Same problem here - new installation for 12.1. After submitting DB user info, etc during the setup process, the following page displays: "Site is temporarily unavailable due to scheduled maintenance, please try again in a minute."

I cannot log into Studio. I have tried the above recommendation from @LeonidS (adding these files BxTemplEditorQuill.php and BxTemplConnection.php) and that didn't affect anything.

Clearly a problem on the source files which has rendered this platform useless for now!! Please get this fixed!!