i did a fresh installation from MyBB on "https://www" which succeeded without errors, but as I tried to login i noticed that the board-URL and admin-URL wasnt working, and the board-folder was on the ftp in the wrong directory, it was in "public_html" instead of desired "private_html". Also the installation url in the edit area in softaulous was pointing to "public_html".
(My solution: I moved the folder manually on the ftp to the private_html dir and edited the url in softaculous, now its ok and I can work with it)