I got this error:
arch_commit: unable to acquire revision lock (could not rename file.)
The problem was the umask of another user committing to the archive. That set the permissions so I couldn't access them.
I figured out what was going on with help from ddaa and Rotty on #arch. They suggested using strace on the tla commit. That helped diagnose the problem.
Then, to fix it, you have to do this (I had to be root for some reason):
Then the commit worked.