Can't upload .msix package in Partner Center console

I have an app available in the Microsoft Store, targeting Windows 10/11.
I've always managed to update it by uploading a .msix package in the Packages section of the Partner Center console.

But for some reason, as of today, I can't upload it. The upload takes forever to start, it doesn't actually upload anything at all, and I get the following error (translated from French): "The mistikee.msix package is taking a long time to process. If it doesn't finish soon, try refreshing the page or deleting the package and reloading it. If this issue persists, contact support."

And I'm stuck because I can't find a way to get in touch with technical support.

How can I get help for that?

Thanks.