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Really the first thing to do is to disable plugins. Normally it's best to take them all off, and add them one at a time until you find the problem.

Since it's admin panel you could do that reverse, disable them one at a time until your admin panel works how it should..

Definitely should cross this off the list before looking any deeper.

Well the error is most likely going to be related to something not getting on with WP..

As Mr Papa suggested updating everything and fixing the older error, wondered if you had tried running just WP and SP since updating and removing the problem. If you have ruled plugins out since updating everything have you tried the WP theme itself? Maybe try default theme and if you have any admin panel themes try reverting to default there as well.

Sorry if you have already tried this, but as I said have to try the basics..