500 Internal Error when Project is opened in separate tab

After migrating from 1.6.1 to 1.7.4 I noticed this strange behaviour:
When I open a project in a separate tab I get a 500 internal error. However, then I simply click the project and it is opened in the same tab, it loads correctly.
Maybe related: After the migration my Flutter-App were rejected. I have to go to Overview - Integrations - Platforms - click on the existing Android-Integration and update it (change the package name and revert the change). after that the app could connect without problems.

Check your browser devtools, for any JS or network errors and please share here.

I have attached a few screenshots. The first one is from Firefox, stating a TypeError: can't access property "account", e.data is undefined. The second one is from Chrome.

You should have upgraded to 1.6.2 and run migrations on that version.
Do you have a backup?

I did. My steps were: 1.6.1 to 1.6.2, run migration and test result, then to 1.7.4, run migration and test result. Yes, I have a backup from 1.6.1.

Where did you run the command? What was the output, were there successful?