@leocg Thank you for the advice. After a month with a clean profile the problem seems to be resolved!

@leocg It's not enabled and never has been.

@leocg
No - Option to clear cookies on exit is enabled
Maybe - How do I fix? - Bad Opera profile directory
No - Opera not being able to write data on the profile directory
No - Third party removing the data

Every week or two I'm logged out of all my WordPress sites, everything Google (YouTube, Gmail, etc.), and other sites. What's going on?

Opera 70.0.3728.144 (and many earlier versions)
Update stream:Stable
System:Mac OS X 10.15.6 64-bit

Block Ads: On
Block Trackers: Off
VPN: Off