Sorry for the trouble with this my friend!
I think that you're already on the latest version (3.5.5), right?
It sounds like our autosave mechanism doesn't play well with your network drive, unfortunately. This is one of the reasons we recently introduced the ability to switch to manual save when needed. Would you mind trying to disable the autosave to see if it helps?
Your changes will still be saved to the internal backup file, and they will automatically recover if you close Mockups without remembering to save.
Hopefully, this didn't corrupt your file but in case it did, please look in those local backups as it should contain a working version of the file.
Hope this helps! Please let us know if this continues to bug you.