Error logging in After Several Log ins and Log outs

This happens when different users on different computers log in and then log out and then try logging in again. The page loads then returns to the login page. From the web console, I see the following error.

I've checked the two source files and it seems that the response body maybe null, what could be the problem?

Does it happen on official release without any modifications? If yes, please provide steps to reproduce the issue.

Haven't tested with official release. But the changes made are not on the server side (java) or the the two files in question. The changes are in the css and images for branding purposes. My server is also just a t2-micro shared between several test projects. Could it have something to do with network and machine performance?

It could be anything. I would recommend to try official release and see if it's reproducible there.

Let me check.