I was attempting to merge a branch back into trunk, however one of the files involved in the merge was opened and locked by another program, so the merge operation failed.
Due to the failed merge, SVN reported that a cleanup was required for any operations, but trying a cleanup resulted in an error along the lines of:
Cleanup failed to process the following paths: C:\Working\Copy Can't open file 'C:\Users\Me\AppData\Local\Temp\svn-<hex value>'. The system cannot find the file specified.
The repository is 14GB, so simply deleting the entire working copy and checking out again is not desirable. Plus I had local changes which would have been tricky to track down without the ability to use SVN to inspect them.
As this is a centralized working copy db (single .svn in the working copy root), there is no option to operate on individual sub-folders to avoid operating on the entire repository.
TortoiseSVN 1.7.10 on Windows 7 x64