.htaccess issue is not related to this plugin. There is a file in your web server root directory called .htaccess, which contains certain page redirection rules, mostly based on regular expressions. If those rules are incorrect, they will cause page redirection problems.
It's basically used to tell the server how to resolve certain requests, like take fancy URLs into a readable format, permalink resolving and so on..
There is very likely an issue there, as the admin-ajax.php file is not accessible, returning a 404 error, very likely caused by a wrong rule. And the site back-end is not accessible either (permanent redirection error), which are almost certainly caused by one or more issue in that file.
If you have not changed anything in that file, then it may come from an incorrect permalink configuration, maybe a cache plugin or anything that has any sort of connection with permalinks.
I'm no .htaccess expert, but I would probably try to re-vert that file to the original contents first, to see if that changes anything. Once that is resolved, I will be actually able to log-in to the back-end, and maybe see the freeze problem as well, as currently the ajax request is not even processed by the server, that has to be resolved first.
If you use an NGINX server instead of Apache, then keep in mind that it does not support .htaccess, and the mod_rewrite rules have to be applied differently. By default WordPress is designed for Apache servers, but I believe there is a tutorial on how to convert it to use on Nginx servers.Best,
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