Turns out the error message was right about having a lock somewhere in the trunk.
Strangely enough, the lock wasn't visible to my client (TortoiseSVN) - neither Release Lock
nor the Cleanup
command reported it. I tried svnadmin lslocks
and lo and behold, it listed a single xml config file locked in... January 2012.
Looks like the upgrade from SVN 1.6 to 1.8 somehow caused this ancient lock to come back. Using svnadmin rmlocks
on that file solved the problem and branching / tagging is no longer failing. Just to make sure I tried locking / unlocking the file through TortoiseSVN and everything works normally now, ie: locks get listed, etc.
Sorry I can't offer a more logical explanation for this problem, except that maybe old versions of locks didn't get upgraded properly, and / or TortoiseSVN doesn't handle upgraded locks correctly.