For some reason the ipa path was changed in the new xamarin beta. Previously the ipa path looked like:
bin/PLATFORM/CONFIGURATION/PROJECT_NAME SOME_TIMESTAMP/PROJECT_NAME.ipa
on the new beta channel the timestamped directory path component disappeared: bin/PLATFORM/CONFIGURATION//PROJECT_NAME.ipa
but the slashes: // are there.
That is why our Xamarin Archive step’s ipa scanner does not find the ipa files.
We are working on the fix 
The Xamarin Builder step works with the beta channel, so as a workaround that can be used.
@godreikrisztian maybe it can also help you to resolve the problem with the Xamarin Archive step?
Definitely, thanks for mentioning!
Another workaround for this can be to switch to another Xamarin stack, the issue only affects the Beta stack. There was also an update for Xamarin Beta a couple of days ago, we’re preparing this week’s updated VMs “as we speak”, should be out tomorrow - we’ll see if the latest Beta changed this.
@godreikrisztian is already working on a fix for the Xcode Archive step too.
This new version(1.3.2) fixes this issue.