that error message is usually indicative of a problem is the comment database.
my suggestion would be to use the File->Export Program feature to export the ladder program and all of the documentation to a text file. look through the text file (with something like notepad) to see if all of your comments are intact. if not, you can fix them in the text file directly.
then close the project and use the Import feature to create a new version of the program from the text file. this will create a new comment database so the problems should be gone.
this may sound like a lot of work for potentially nothing that shows up onscreen, for now, but if the comment database is corrupted, you will eventually do something that will cause damage, potentially losing all of your comments.
better safe than comment-less.