Hi @neso, the Discourse._registerPluginScriptDefinition is not a function error was fixed in Revert plugin js changes (#5139) · discourse/discourse@7d350d0 · GitHub last week.
In order to access the admin/upgrade page again, you’ll need to SSH into your server and run
cd /var/discourse
./launcher rebuild app
Here’s the topic from last week:
https://meta.discourse.org/t/fatal-js-error-breaking-admin-upgrade-page/69559?u=david_taylor