Oftentimes, error No916 is caused by an issue with file integrity.
What operating system are you using? If you're using Windows 10, try adding an exclusion in Windows Security. PSOBB is notorious for triggering 'false-positives' which Windows Security removes without warning or consent.
1) Navigate to 'Windows Security' either by using the search function or by copy/pasting "windowsdefender://threat/" into Run (using the latter will take you to step 3).
2) Go to 'Virus & threat protection' on the left panel.
3) Under 'Virus & threat protection settings' go to 'Manage settings'.
4) Under 'Exclusions' go to 'Add or remove exclusions'
5) From there click on 'Add an exclusion', select 'Folder' and add the location of your 'Ultima PSOBB' folder.
6) Reinstall Ultima PSOBB to the same location that you added the exclusion. (Download page)