Presentation slides are not visible during meeting, but in recording ...

[tobiasjj]

I have installed BigBlueButton 2.2.31 as described in README.md. The docker containers run behind an apache2 proxy and a firewall with the necessary ports opened. Everything runs fine (audio, video, screensharing, recording, ...), and recorded presentations show all the slides. However, during the meeting presentations are not visible, only the controls are (and functional):

I can not identify any relevant error message, when initializing a session and subsequently trying to present the default.pdf, while monitoring with docker exec bigbluebutton_core_1 tail -f /var/log/bigbluebutton/bbb-web.log (anonymized):

...
2021-02-16T10:40:40.613Z DEBUG o.b.web.controllers.ApiController - ApiController#uploadDocuments(XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX)
2021-02-16T10:40:40.618Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"duration":0,"record":true,"logCode":"create_meeting","name":"Home Room","description":"Create meeting.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi","isBreakout":false,"webcamsOnlyForModerator":false}
2021-02-16T10:40:40.622Z DEBUG o.b.web.controllers.ApiController - ApiController#downloadAndProcessDocument(https://SERVER.DOMAIN.NAME/default.pdf, XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX, )
2021-02-16T10:40:40.657Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"duration":0,"record":true,"logCode":"meeting_started","name":"Home Room","description":"Meeting has started.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi","isBreakout":false}
2021-02-16T10:40:40.880Z DEBUG o.b.web.controllers.ApiController - Rendering as xml
2021-02-16T10:40:41.062Z DEBUG o.b.web.controllers.ApiController - ApiController#join
2021-02-16T10:40:41.064Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=082e01bfc93d864f43bd8bf7da95e014b76399f0 length=40
2021-02-16T10:40:41.065Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=082e01bfc93d864f43bd8bf7da95e014b76399f0 length=40
2021-02-16T10:40:41.066Z INFO  o.b.web.controllers.ApiController - Retrieving meeting 1a0530207665392721c585041e310567eac326eb
2021-02-16T10:40:41.067Z INFO  o.b.web.controllers.ApiController - Session sessionToken for USERNAME [null]
2021-02-16T10:40:41.072Z INFO  o.b.web.controllers.ApiController - Session user-token for USERNAME [null]
2021-02-16T10:40:41.073Z INFO  o.b.web.controllers.ApiController -  --analytics-- data={"extMeetingid":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi","logCode":"join_api","name":"USERNAME","sessionToken":"XXXXXXXXXXXX","description":"Handle JOIN API.","meetingid":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","userid":"X_XXXXXXXXXXXX"}
2021-02-16T10:40:41.074Z INFO  o.b.web.controllers.ApiController - Redirecting to https://SERVER.DOMAIN.NAME/html5client/join?sessionToken=XXXXXXXXXXXX
2021-02-16T10:40:42.441Z DEBUG o.b.web.controllers.ApiController - ApiController#enter
2021-02-16T10:40:42.442Z INFO  o.b.web.controllers.ApiController - Token XXXXXXXXXXXX is valid
2021-02-16T10:40:42.442Z INFO  o.b.web.controllers.ApiController -  --analytics-- data={"extMeetingid":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi","logCode":"handle_enter_api","name":"USERNAME","sessionToken":"XXXXXXXXXXXX","description":"Handling ENTER API.","meetingid":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","userid":"X_XXXXXXXXXXXX"}
2021-02-16T10:40:42.526Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"role":"MODERATOR","clientType":"HTML5","externalUserId":"gl-fspvqctupvnz","logCode":"user_joined_message","name":"Home Room","description":"User joined the meeting.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi","guest":false,"guestStatus":"ALLOW","userId":"X_XXXXXXXXXXXX","username":"USERNAME"}
2021-02-16T10:40:44.476Z DEBUG o.b.web.controllers.ApiController - ApiController#stuns
2021-02-16T10:40:44.477Z INFO  o.b.web.controllers.ApiController - Token XXXXXXXXXXXX is valid
2021-02-16T10:40:44.477Z INFO  o.b.w.services.turn.StunTurnService - 
StunTurnService::getStunServers 

2021-02-16T10:40:44.477Z INFO  o.b.w.services.turn.StunTurnService - 
StunTurnService::getStunAndTurnServersFor X_XXXXXXXXXXXX

2021-02-16T10:40:45.879Z INFO  o.b.p.DocumentConversionServiceImp -  --analytics-- data={"current":true,"filename":"default.pdf","logCode":"presentation_conversion_start","authzToken":"preupload-download-authz-token","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","message":"Start presentation conversion.","podId":"DEFAULT_PRESENTATION_POD","presId":"YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY-YYYYYYYYYYYYY"}
2021-02-16T10:40:45.880Z INFO  o.b.p.SupportedDocumentFilter - Received supported file /var/bigbluebutton/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX/YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY-YYYYYYYYYYYYY/YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY-YYYYYYYYYYYYY.pdf
2021-02-16T10:40:46.242Z INFO  o.b.p.imp.PresentationFileProcessor -  --analytics-- data={"filename":"default.pdf","num_pages":6,"logCode":"presentation_conversion_num_pages","authzToken":"preupload-download-authz-token","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","message":"Presentation conversion number of pages.","podId":"DEFAULT_PRESENTATION_POD","presId":"YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY-YYYYYYYYYYYYY"}
2021-02-16T10:40:49.396Z INFO  o.b.p.i.PresentationConversionCompletionService -  --analytics-- data={"current":true,"filename":"default.pdf","logCode":"presentation_conversion_end","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","message":"End presentation conversion.","podId":"DEFAULT_PRESENTATION_POD","presId":"YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY-YYYYYYYYYYYYY"}
2021-02-16T10:41:21.047Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"duration":0,"record":true,"logCode":"meeting_ended","name":"Home Room","description":"Meeting has ended.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi"}
2021-02-16T10:41:21.047Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"logCode":"kick_off_ingest_and_processing","name":"Home Room","description":"Start processing of recording.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi"}
2021-02-16T10:41:21.048Z INFO  o.bigbluebutton.api.MeetingService -  --analytics-- data={"duration":0,"record":true,"logCode":"meeting_removed_from_running","name":"Home Room","description":"Meeting removed from list of running meetings.","meetingId":"XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-XXXXXXXXXXXXX","externalMeetingId":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi"}
2021-02-16T10:41:23.963Z DEBUG o.b.web.controllers.ApiController - ApiController#isMeetingRunning
2021-02-16T10:41:23.965Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=96794307a802f3d84ef3a0bba7adbbc2015cd99f length=40
2021-02-16T10:41:23.965Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=96794307a802f3d84ef3a0bba7adbbc2015cd99f length=40
2021-02-16T10:41:23.965Z INFO  o.b.web.controllers.ApiController - Retrieving meeting 1a0530207665392721c585041e310567eac326eb
2021-02-16T10:41:23.992Z DEBUG o.b.web.controllers.ApiController - ApiController#getRecordings
2021-02-16T10:41:23.993Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=b608e1c1e721b7fbbb1c566cdc6f048ffc015b8b length=40
2021-02-16T10:41:23.993Z DEBUG o.b.web.controllers.ApiController - meetingID=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxi&checksum=b608e1c1e721b7fbbb1c566cdc6f048ffc015b8b
2021-02-16T10:41:23.993Z INFO  o.b.api.ParamsProcessorUtil - CHECKSUM=b608e1c1e721b7fbbb1c566cdc6f048ffc015b8b length=40
2021-02-16T10:41:23.993Z DEBUG o.b.web.controllers.ApiController - 1a0530207665392721c585041e310567eac326eb
...

Do you have any ideas?
Thanks for the great work!