Request failed while trying to migrate

I want to migrate a self hosted appwrite instance to another one. When i enter the source data in the new appwrite instance and click next and wait a minute, i just get the error message Request failed: Please check if your credentials are filled in correctly in the previous step In the Chrome Developer Console i can see the error 504 (Gateway Time-out)

I copied the parameters directly from the overview tab (the .../v1 endpoint and the project ID) in the old instance and also created a new API Key with all scopes, that i copied over directly...

Is there any possibility that the error message is wrong in the assumption, that the credentials are wrong and that there is an alternative reason? If so what would be some steps to find and or fix the issue?

Thanks in advance for any help