500 errors on wp-admin, login page

I’m using plugin Version 1.8.20 on wp version 5.2.1

Upon login I receive 500 internal error. I can then remove wp-admin from the url and navigate to the homepage. There I can get into my dashboard. I try to upgrade latest version of wp and receive error. And it will not upgrade, I get error page. I try to deactivate plugins and I get error. I manually disable plugin from ftp and all is working correctly. When I tried to restore my blog to a later point before manually disabling sucuri plugin it will work but I get an error in ManageWp dashboard. It is the following.

The process finished, but we got an error when we attempted to create the .htaccess file: E_ERROR: Allowed memory size of 268435456 bytes exhausted (tried to allocate 75 bytes) in /home/WEBSITE/public_html/WEBSITE.com/wp-content/plugins/sucuri-scanner/src/fileinfo.lib.php on line 431.
You can complete the process manually by going to the destination’s WordPress admin dashboard, then to “Settings > General” and clicking “Save Changes”.

Please advise why sucuri is breaking my blog. Thank you.