I have a wild and crazy idea before we try too hard to fix this. Just clear the cache on your browser. That isn't what the error is telling me, but updates can do some crazy things sometimes.
So, since you're still here the cache thing didn't work. We need to disable Admin Tools with the instructions here:
https://www.akeebabackup.com/documentation/troubleshooter/atwafissues.html. That should allow you to log into the back end of your site. Now we want to install Admin Tools twice, back to back without doing anything else in between. This trick usually works around a partial install of the component, caused by a bug in the Joomla! installer that goes back many years. Many have searched, but nobody has found it - yet.
Since the error is in the FOF library, the double install of Admin Tools may not work. You may need to download the FOF30 library from our Downloads page and install it directly. The FOF library is really tricky when you have more than one Akeeba product installed.
Have a happy Labor Day!
Dale L. Brackin
Support Specialist
English: native
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
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My time zone is
EST (UTC -5) (click here to see my current time in Philadelphia, PA)