Quick update for this topic ...
I had exactly the same problem after updating LBC, I use portable versions and in explicitly named directories, eg c:\programs\lbc_141 after an upgrade to new version and a change of directory c:\programs\lbc_144, I copied my MCF files to new directory, then DELETED the old directory, and then hit this same error.
It seems that one of the the mcf filenames was being displayed by LBC, and could be edited, but was pointing to the old directory, so of course could not be saved.
Thanks Mouser for this supper software. ( APP ??? )