I get also this error message since I upgraded from 1.7.16 to 2.04.
The message pops up (most of the times, not always) when I do the Login. The fault appears, then I restart the website and I am loged-in.
I never encountered it on other actions/pages.
Didn't mentioned it before because I could use WT, but I guess that it is similar to what HiBratt has noticed.
Now it happens al the time. No problem with other sites. Tested with Chrome and Firefox. I also get the message test to clean cookies. I ahve two cookies related to the site both with the name WT2_SESSION. Chrome Console reports "DevTools failed to load SourceMap: Could not load content for
: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE"