Mac AppStore version of Enpass 5.6.11 is available too, and I can confirmed it fixes the issue with Opera. Go Update!
!! Remember to Check/Enable the Verify browsers setting from the Enpass app if you've previous disabled it to workaround the bug.
I too encountered the same problem after upgrading. I'm now on Opera 53.0.2907.68 on macOS 10.13.4
I found if you disable the Verify browsers setting in the Enpass desktop client, the browser extension will work again. Maybe the browser extension code signing expired?
At lease you can get it work... hope that helps.