Joomla has definitely messed up the update of your site. Your error message means that files are missing from libraries/fof30. That's where FOF 3, the common library for all of our components, is installed. However, this common library is NOT reinstalled when doing the double component package installation because the package is told by Joomla that the library is already installed and in its latest version.
There is a fix to that.
Download the latest version of FOF 3 from https://www.akeeba.com/download/fof3/3-7-1.html The latest version is towards the top of the page.
Install that ZIP file twice in a row. Now the problem should be solved.
PS: I have confirmed that the problem is not in what we published by doing two tests. First, I created a brand new J 3.9.23 site on PHP 7.4.13 and installed Akeeba Backup Pro 7.5.0 on it. No problem. The second test was an existing, live, J 3.9.23 site on PHP 7.4.13 (my blog, that's how confident I was it's not going to cause a problem) which upgraded from Akeeba Backup Pro 7.4.0.1 to Akeeba Backup Pro 7.5.0 using Joomla's extensions update. The first test makes sure that there's nothing broken in the package we released. The latter test makes sure that I didn't fat-finger something in the XML manifest which could have broken the update but not the clean installation. That's on top of all of the other tests I had done before the release (being paranoid and testing again never hurt anyone!).
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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