Unfixable mixed-revision working copy in SVN

Note that this issue has been fixed since Subversion 1.7.0. Since then svnversion (and the merge logic) ignore file externals when determining the revisions.

Looks like you're not the only one with this problem.

The fix is to use the equivalent of svn update --ignore-externals

See here: Reintegrate a branch with externals fails in SVN

Even immediately after updating/pushing my entire repository, I got this error message just for trying to rename a folder. Updating the folder again right before renaming fixed this issue for me.

I came to a different solution, --allow-mixed-revisions:

svn merge -c7777 https://repo/app/branches/1.0 . --allow-mixed-revisions

Ignoring externals when updating didn't fix the problem for me even if I did a completely fresh checkout.