Was there some Windows update or OneDrive update recently? AccessDeniedException means that Cryptomator is unable to write a file (in this case, it can’t backup the masterkey).
We’ve just received another (identical) report here: Unlock failed see log file for details (One Drive)
Did Microsoft roll out the “Files On-Demand” feature for everyone? Or are you using Windows Insider Preview? Please check out this thread and see if that’s the case: Unable to use OneDrive with Files On-Demand enabled