Hello Christian,
The problem you are experiencing is caused by Joomla!'s JUpdate core API which is responsible for installing extensions. Two and a half years ago I filed a bug report to the Joomla! project along with a patch which would at least make it possible to work around the issue by reinstalling an extension without uninstalling it first. My patch was rejected because they said it was a bandaid and a better solution would be written soon. I don't know how they define "soon", but after more than two years having gone by without a trace of activity I guess they use a cosmological time scale.
Perhaps you will be lucky and won't have to go through the entire database surgery procedure described in our troubleshooter. Please try installing Admin Tools again, without uninstalling it first. Do that twice in a row (yes, it's odd, but it's important). I have put some parts of my patch in the post-installation script that runs right after JUpdate installs a component on your site. It seems to work in 90% of similar problems, but only if you try doing the reinstallation twice in a row.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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