Your message implies that you can save some reports to prf but not others.
Is that the case? If so ...
What sort of error do you get for those that will not save?
Are there any obvious differences between the files that save and the ones that don't?
Are then any file and folder access and permissions constraints on your system?
Did anything else change at around the same time as your Monarch upgrade?
Are you the only user of Monarch? If not do any of the other users have the same problem?
Let's see where the answers to these questions lead us.