I have had this problem in the past with 2012 and as far back as 2005. I really don't know what is going on; but, it is frustrating.
My fix is to create a new try and import from latest backup.
Personally, I don't think it has anything to do with hidden files. It has to do with an internal switch in the file is being set to say that is already open. It is done this way so that multiple users on a network can't open the same file at the same time and make changes over each other. Somehome, the switch is not being turned off or reset. I have noticed that the last time it happened, I had left FTM open and Windows did an automatic shutdown and reboot after updating from Microsoft. I think that the file was improperly exited and I couldn't get back into it even if I copied the file from my Windows computer to a Linux network and back to my Windows computer hoping that the switch would reset...and it didn't.
So, I would recommend multiple manual backups and do not leave FTM open when not in use.
My fix is to create a new try and import from latest backup.
Personally, I don't think it has anything to do with hidden files. It has to do with an internal switch in the file is being set to say that is already open. It is done this way so that multiple users on a network can't open the same file at the same time and make changes over each other. Somehome, the switch is not being turned off or reset. I have noticed that the last time it happened, I had left FTM open and Windows did an automatic shutdown and reboot after updating from Microsoft. I think that the file was improperly exited and I couldn't get back into it even if I copied the file from my Windows computer to a Linux network and back to my Windows computer hoping that the switch would reset...and it didn't.
So, I would recommend multiple manual backups and do not leave FTM open when not in use.