Upgrading to v. 3.6.6.3 seems to have partially broken the MM functionality which triggers the display of the "This is an evaluation version of Markdown Monster..." banner. I paid for an upgrade license from v2 to v3 about 1.5 years ago, as can be seen from this screenshot:
Additionally, the View | Internal Preview functionality no longer works.

Yeah, that doesn't look right, but I can't duplicate this... your license looks good, so it's not that.
The preview not showing is likely because it looks like you are in Distraction Free Mode and probably have the previewer in the list of hidden components. Click on the Maximize (two diagonal arrows) icon in the Control box of the window to get out of DF mode if you're there by accident.
Would you mind filing an issue on GitHub so we can track this to see why this might be happening? I can follow up there.
Markdown Monster Bug/Feature Request Reporting
FWIW, I can't duplicate that even trying to map my DF mode similar to yours - I don't see the banner once registration is active. The banner should only show if the registration does not validate, but it sure looks like the app validated just fine as it shows on the about page.
Obvious question: Did you restart MM at some point to see if the behavior is different? And did you try unregister and re-register.
The latest full installer version of MM wipes the entire install folder completely, so if you had an old registration the registration file ended up in that folder and probably got wiped out. The register file has since moved to a separate location in %localappdata%\Markdown Monster
. That might account for some of the registration weirdness. If you re-register the file will go to the new location.
Unregistering and reregistering should take care of that if that's part of the problem and if there's a problem with permissions or the file can otherwise not be written you should get an error with more info during the registration process.
+++ Rick ---
OK Mark, I've raised issue #1181 for this problem (the faulty evaluation banner problem has disappeared).