No route found for path: "undefined" and request method: "GET"

Hi, I’m migrating my site from Kirby 3.x to Kirby 4.1.
I get this error on the panel:

No route found for path: “undefined” and request method: “GET”

What can I check?
Thanks

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Just in case some other guys get into the same error - it was a markdown field plugin. Just remove the plugin.