I've updated wordpress to the 5.5 version, how can I remove this?
I've an empty div which has a margin-top: 2rem, which causes the white space on top, but can't explain for the footer
<div id="adminmenuback"></div>
According to this post: How to fix the admin menu margin-top bug in WordPress 5.5?
There is no solution yet, but maybe this will help you find one.
Personally, I tested locally and on a SiteGround server, but I can't recreate the problem. However, if you can tell yourself it's a PHP error like the link I gave you above, maybe your file with PHP log errors will tell you something.
(Sorry for my bad english, j'ai voulu t'écrire en français, mais bon ^^)
Don't forget to write define('WP_DEBUG_LOG', true) in your wp-config file.