Apple AppStore is unable to process purchaes

Hi,

starting some days ago I noticed that all of my iDevices have the same error when updating or downloading an application from the AppStore.
The message says "The iTunes Store is unable to process purchaes at this time. Please try again late".

First, I though Apple has some disruption with their servers. But as the error persists today I started digging.
I noticed that this error only occurs, when the devices are connected via Wifi and the Sophos XG with the internet. If I disable wifi on the devices, I can download or update any app via LTE.
So I reasoned that the problem must be related to the Sophos XG.

I cannot see any suspicous log statement. I even disabled web filtering completely - still the same error.
I am not sure if this is related, but the Sophos sometimes reboots without any obvious reason. This started some weeks ago. I searched the web log viewer and also the log files via SSH, but can't find any hint. It seems that the Sophos XG crashes and reboots automatically.

I have not changed any configuration for month. The Sophos ran smooth until it started rebooting ... 

Can you give me some hints, how to figure out what's causing the problem with the AppStore and why the Sophos is rebooting.

Thanks for your help

EDIT:

I did some tests earlier today and it seems that the error only occurs, when I am going to update an app. If I buy a new one, the download starts after authentication.