Upgrading from 3.8.11 to 5.3.1 failed somewhere

[somasimple]

Hi,

The upgrading process went well but failed at end.
It was impossible to connect to the admin cp (access denied).
The forum is now a mess

https://www.somasimple.com/forums/

When I try to access the admin cp via this URL it says NOT FOUND 404

https://www.somasimple.com/forums/admincp/index.php

When I try with

https://www.somasimple.com/forums/core/admincp/index.php

it give a good login box but sends to a NOT FOUND, too...

Any help will be appreciated.

[somasimple]

I found an error in the htaccess file in the forums folder (no point at the beginning)?
Correcting this problem makes a TOO MANY REDIRECT ERROR.

[somasimple]

SOLVED => problem with the rewriting rule of https.
Well albums are empty as all thumbnails of styles ???

[somasimple]

And some problem while posting =>

[somasimple]

Lost signatures too?