os application open denied

Boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Log into another vanilla admin user account, Set up users, guests, and groups on Mac,  run all three apps for a while and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

If the problem goes away in the other user account it means there's something in your account that's the culprit. If that's case download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Then I would contact Apple and talk to them about it and see what they say. If you don't the developers will just pass the buck again. As mentioned its up to them to make their programs compatible with the OS. In the meantime maybe someone will see this thread and offer a solution that worked for them. Cheers.

In my view its an OSX issue.

Only happens with non-native apps. All of these, on my machine, have permissions etc.

According to the Apple Advisor I went through all possible troubleshooting they had.

It is definitely a known issue. Apple is pointing to developers but IMHO this is not quite correct.

You should contact the developers. OmniOutliner was just updated a couple days ago.

I can’t help you as to why but since they are third party apps I’d also contact their respective help desks and ask them.

I can't say since I don't use any of the apps. I don't know why they would suggest contacting Apple since it's up to them to make sure their apps are compatible with Big Sur.

👍