ERROR - Class 'Grav\Plugin\Login\Controller' not found

Not sure if i asked this already but please PM me a link to a ZIP of your site, and i'll try locally.

@bdillahu zip and send us (via Gitter) your user/plugins/login folder to check why is that happening.

The error is, for those having the same, during an update the user/plugins/login/classes/controller.php was renamed to user/plugins/login/classes/Controller.php (notice the capital C).
Probably the GPM update didn't rename the file, considering it the same filename but on case-sensitive filesystems (Mac, Linux) it's not.

So to solve this, manually rename user/plugins/login/classes/controller.php to user/plugins/login/classes/Controller.php

Yeah! I think that was it...

And with things in git (sorry, I'm not the strongest git user), the lower case "controller.php" was "hidden" or eclipsed or something.

Doing a:

git mv -f controller.php Controller.php

worked, even though you couldn't see a controller.php in existence. Maybe this is a quirk of a Mac's filesystem (I'm more used to a Linux box for this kind of thing).

Thanks again!

Renaming the controller.php as @flaviocopes suggested worked for me