Using beta 12.28.2 whenever I double click on a .mp4 file (or choose OPEN from a right click) dOpus throws an error "The specified procedure could not be found." before opening Media Player (default). My associations are set in Windows Setup/APPS and double clicking in Windows Explorer opens the file correctly. Using Windows 11 Pro 21H2 build 22000.832.
UPDATE: False alarm! Rebooting computer solved issue. (Restarting dOpus may have had a similar result). Thanks! Strange because the dOpus icons correctly reflected the association...