Hello,
It might be an issue with a plugin.
Can your client try the safe mode? He should be able to access the admin panel at least.
I think more details are needed to understand what’s happening.
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If it’s an upgrade from the panel or the command line, I would try it from the command line to see if it helps upgrade everything.
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Are there errors in the browser’s console? This can help to figure out the culprit if it persists. (right click on the page → inspect → Console tab → copy-paste or screenshot the content)