Panel error "Exception: undefined Class name must be a valid object or a string"

Kirby 3 Questions 🤔
1

I haven’t used Kirby for a while and updated it recently to the latest version including plugins.
When I want to write a new article I get this error after clicking on save:

Exception: undefined
Class name must be a valid object or a string

The browser console shows a lot of 404 and 500 errors which haven’t been there last time I used Kirby (AFAIK):

image
image1666Ă—1145 171 KB

How can I solve this issue?

2

Your Panel looks customized regarding translations and there are error messages pointing to translations as well, so I’d look into that. Not knowing what plugins or custom code you are using here, hard to tell.