I tried to digitally sign a .xlam (Excel Macro-Enabled Add-In) but no matter what modifications I did to the file the changes were not saved. Even if I put a copy of it in a different folder it did not help. I tried to disable all Add-ins, restarted Excel and tried to modify and save to no avail.
Have you seen this behavior? If so, I was able to solve it by starting Excel in safe mode (holding Ctrl down while starting Excel) and then open the .xlam file, modify and sign it.