I had the exact same problem and I managed to find a solution. The error was caused by the latest version (2.6.0) of the Microsoft.UI.Xaml NuGet package. After downgrading to the previous version (2.5.0) the certification run without any problems.
Application certification issue

Salavat
121
Reputation points
I sent the application for certification and the first few attempts ended with the error "A previous submission failed certification." Then the submission was successful, but the certification got stuck at the "Pre-processing" stage, attempts to cancel through the "Cancel certification" failed.
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