Recently I have been unable to make a new Lister format stick as default (it worked before). The only change I know if was a new display resolution (4K) and setting up a secondary screen (which is not actually used - it's just a lower resolution HDMI input backup). Not sure if or why this is a factor. I've tried fully closing and reopening, rebooting, etc. Version is 10.5.7.0, win 7 64 bit. Any ideas much appreciated!
The Startup Folders FAQ should list the things you might need to do.
If you're still stuck, please tell us the settings you have under the pages below Preferences / Launching Opus, and also link your account.