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This problem is unrelated to Admin Tools. Joomla displays this message when there are no published plugins in the authentication group or when none of the published plugins can load.

If this happened immediately after installing Admin Tools (before you even had the chance to see the confirmation message that the extension was installed) it's caused by a problem with your server, most likely the storage's filesystem is corrupt; you have to ask your host to help with that.

If this happened sometime after you had already installed Admin Tools it's unrelated to Admin Tools at all and very likely to be the result of a change you made to your site. We definitely don't mess with Joomla's authentication plugins, ever. I know full well exactly the problems such a course can cause  and made sure we steer clear of any change related to the authentication plugins. For what it's worth, next week I'm having a call with the Joomla Security Strike Team to help improve the Joomla authentication stack to make it safer and more resilient — that is to say, I don't just know better than to mess with Joomla's authentication plugins in my software, I also know full well what can cause them to bork and I'm actively trying to help Joomla address these issues.

Yes, that would do it. That's why I mentioned the corrupt filesystem. In some cases if the account storage is full you might get corrupt files. I've only seen it personally happen with FTP (which is not available on Joomla 4) but I've also been told it could theoretically happen with a direct file write under some circumstances.