I have made some progress on this matter. I hope this can help others to go further than I have managed so far.
I was flicking through my ZoneAlarm Pro logs and I noticed three alerts for 'OSFirewall'. They were all for Messenger Plus, where ZoneAlarm had blocked it opening notepad as another process.
I disabled OSFirewall and restarted the system. The decrypted file error is now gone. BUT...when opening the file, I now get the error from Notepad (not MSGPlus):
"The filename, directory name, or volume label syntax is incorrect."
It opens a blank notepad window to go with this. My limited research on this subject has turned up a bunch of sutff about hidden streams, but nothing I can make progress with. I'll keep trying and will report with any developments.