This is a nasty one... mysterious error and no clear fix.
update/revert/commit did NOT work in my situation. I hadn't done anything weird - just some svn moves.
What DID work for me was:
svn remove offender
svn commit
cd ..
rm -fR parent
svn up parent
cd parent
svn remove offender again
svn commit
copy offender back in (minus .svn dirs)
svn add
svn commit
Weird to say the least. Basically, the svn remove --force offender
wasn't doing completely removing for some reason. Which is sort of what the error message was saying. Only by removing the parent, then updating the parent, did this become obvious because then the offender reappeared! svn removing offender again then properly removed it.