That can be something hard to catch if you don’t know where to look!
I spent some time on this issue – SonarQube’s End Analysis task fails, with either an unhandled exception (version 2.1.1 of the extension) or a 404 page in plain text in the log (with the older 2.0.0).
What was really odd was that the issue happened only on certain projects – some were fine, some others were failing, And it happened regardless of the build server used by TFS – whatever the agent, this randomness was there.
At the end of the day, don’t underestimate logs collected from the Web Server you are using: I found an error 404.13 in the logs, meaning the Analysis Report was exceeding the size limit for the upload.