How to fix it? Version 1.6.5
Since you wrote "General Error", no screenshot or any further information before or after the "General Error" . Then my first suggestion is to update your app.Why update? How is it related to the problem?
{"errors":{"license_key":"This license is already registered to another MailWizz instance"},"message":"General error","status":"error"}
It is. There are some changing to license checking API which are addressed in the newer versions. Please read: https://www.mailwizz.com/kb/this-license-is-already-registered-to-another-mailwizz-instance/Why update? How is it related to the problem?
That is not ok. That means you will modify the core files and we cannot offer support if you go that wayOk, you say to update - I disabled it completely including part where it moved app offline.
You say it can be a license - I'll disable the license check as well if the previous one won't help.