Guideline 2.1 - Performance - App Completeness

I'm made an application in flutter. I've uploaded several updates to the app without any problems, last one a few days ago. Now yesterday i tried another update and it got rejected with this message:

We discovered one or more bugs in your app when reviewed on iPad running iOS 14.0 on Wi-Fi.

Apple login was not responsive.

I have no clue what this is but as far as i can see, this has been a problems for a while with other users. Thing is, it works perfectly on physical devices. Any input is much appreciated.
Guideline 2.1 - Performance - App Completeness
 
 
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