Several times this nasty bug has bitten me, and I assume I’m not the only one :-).
Are you having problems with IntelliJ IDEA refusing to authenticate you while trying to use SVN (TortoiseSVN)? You have even tried to clear the authentication cache and all other logs?
If so, you might try to add the following to your idea.vmoptions (idea.exe.vmoptions if you’re using Windows) file:
-Dsvnkit.http.methods=Basic,NTLM
Restart your development environment, and everything should be back to normal.
This issue has also been addressed several times in the JetBrains.net Developer Community, where JetBrains developer Serge Baranov has most, if not all, answers.
Happy hacking!