Sadly this is beyond me and hopefully someone more knowledgeable comes along soon, I don't see anything but I'm not an expert. When was the last time you made an edit to PM?
Restore from a backup in the Backups\Automatic folder before you started testing that. Hopefully it won't happen again but it generally happens when the deserialise fails for some reason. In this case it wants to delete the player mobile that threw the error.
When I say delete, it deletes all the characters, but after installing the previous backup, it continues without any problems. He does it occasionally. I rebooted 50 times in 3 days but it didn't happen again. During this time I did not remove or add anything from playermobile.cs. Very strange