SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

[lkbhitesh07]

This error occurred recently in test server:

Frontend error: 
Uncaught (in promise): SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
    at JSON.parse (<anonymous>)
    at XMLHttpRequest.a (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:3536:1456)
    at e.invokeTask (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1958:7859)
    at Object.onInvokeTask (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1377:1589)
    at e.invokeTask (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1958:7780)
    at t.runTask (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1958:2934)
    at t.invokeTask [as invoke] (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1958:9015)
    at p (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1989:733)
    at XMLHttpRequest.d (https://www.oppiatestserver.org/build/webpack_bundles/vendors~lightweight_oppia_root~oppia_root.386d26d6d006ef953b2b.bundle.js:1989:1051)

    at URL: https://www.oppiatestserver.org/logout

Where did the error occur? https://www.oppiatestserver.org/logout

Which release did the error occur in? 3-3-9

Frequency of occurrence Once in the past 4 days(8 May).

General instructions for contributors
In general, the procedure for fixing server errors should be the following:

• Analyze the code in the file where the error occurred and come up with a hypothesis for the reason.
• Based on your hypothesis, determine a list of steps that reliably reproduce the issue (or confirm any repro instructions that have been provided). For example, if your hypothesis is that the issue arises due to a delay in a response from the backend server, try to change the code so that the backend server always has a delay, and see if the error then shows up 100% of the time on your local machine.
• Explain your proposed fix, the logic behind it, and any other findings/context you have on this thread. You can also link to a debugging doc if you prefer.
• Get your approach validated by an Oppia team member.
• Make a PR that fixes the issue.