The error I spoke of actually appears when I click on the Rename button, not only the Edit button. So that did not work for me.
The solution I figured out was to use a binary editor to edit the .VBF and .VB~ files to change the name in the XML code. That worked for me. Notepad did not work as it destroyed the binary form data.
Backups of forms (using export) and copying the VBF files is now a regular part of my work process. 🙂