I managed to get my install to go back to being "OPEN" by going to Add-Ons and uninstalling the PRO addon. It now says OPEN on the login screen and in the bottom right when logged in.

However, I am still being prompted with a red "you don't have a license" screen on every page of the site. Everything works as it should, except for this extraneous red modal.

I am experiencing the same issue with this. I managed to fix the database error (change the mysql database user in config.inc.php to idoit instead of root), but it has set itself up as PRO, when I selected the community/open edition.