svn commit fails: File not found: Transaction »52-1r«

Short Answer

My client version of subversion was outdated.

Long Answer

I'm running MacPorts on my Mac and somehow assumed that I had Subversion installed as a part of that. Alas, I had not, and my svn command was Apple's default svn, version 1.6.17 (r1128011) which runs with subversion format 10.

The repository runs svn, version 1.7.13 (r1516569) which runs with subversion format 12.

After updating (i.e. installing) my local svn, version 1.8.3 (r1516576) which runs with format 12 as well and svn upgrade my local repository tree to the current version, I could commit all my changes without problems.

This is resolved for me after selecting check box present at bottom of SVN client. "Create intermediate folders"

You will also get this message when creating a tag using the client by copying into a new path where more than one level of the desired folder structure does not exist in repository. (Jeez that sounds complicated.)

In other words, I attempted to create a tag:

/projects/MyAwesomeProject/tags/myFirstTag

...but the 'tags' folder did not exist yet. I expected the client to do the work for me; manually creating the 'tags' folder was the cure.