Yeah, this is happening because we're currently cleaning up the library and renaming a lot of template files. When it's done, old template filenames will automatically translate to the new ones, so those errors should be gone after several nightly updates (when all filenames are finalized). For now we suggest not opening files created with stable builds in the nightly (or at least not saving the file once that alert has been shown).
Sorry for the inconvenience, and please look forward for the progress!