To save someone time in the future:
If you are fixing a hook in a Subversion repository, such as post-commit, and your clients get the message “Warning: ‘post-commit’ hook failed with error output:” and nothing else, it means that the script exited with a nonzero error status but did not print anything to standard error. However, this is also the effect of attempting to execute a non-executable file, and Subversion does not mention to you if your hook is not set executable. The first thing you should check is whether it is.
It also happens if it is a script that is executable by you but not readable, as I found with one project I worked with.
The outcome in this situation is that the commit has succeeded but your working tree is still pointing at the old revisiion. A simple “svn update” seems enough to get you going again (although the post-commit won’t have run).