Hello Michael,
that plugin is no longer useful nor maintained, this usually happens when you try to update from a very old release of Admin Tools.
Please unpublish it, it will be automatically removed on later versions.
In the meanwhile, to gain access to your site, manually rename the atoolsupdatecheck folder using FTP, so the plugin won't fire and you'll be able to connect to your site.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!