anybody seeing issues with XG17 causing Outlook desktop client losing "sync" with Office365.

We have the exact same issue.  I read below that it may be contributed to Load Balancing but we don`t have multiple WAN links and we are still getting the problem. We have been dealing with this since we moved to Sophos about 5 months ago. I have tried Firewall Rules, Web Exceptions just about everything.  If i disable all scanning and use the XG as a router it seems to clear up.  I haven`t left it with everything turned off long enough to know that is the case because it would defeat the purpose of having a firewall in the first place.

We also had issues with the office straming install that was the HTTP scanning. So we created a new firewall rule for that and outlook access to O365. Turned off scanning and changed it from load balancing to a single egress and failover. This was on Friday I believe. Because the issue is so completely random we are giving it a good week to see if it rears its ugly head again.

this is what I have set now, with no positive effect.

I am still working with our SD-WAN provider to see if it is anything on their end.

Try turning off load balancing. That has helped us so far.

Load Balancing Off and my CFO just told me he quit using outlook and has gone online because the issues are so bad for him.  Like I have stated before I have been battling this issue for over five months. The day we put sophos XG in place we have had nothing but nightmare issues.  We will not renew Sophos My personal Opinion is their sales pitch does not meet their product delivery.  

This is my first exposure to Sophos. I had strictly used Netscreen/Juniper and Cisco SA and ASA's. Much more robust and none of these wierd traffic issues. I know as we get into endpoint protection and chewing apart every bit of a packet to find that PITA malware or ransomeware, that the network will be impacted. But for it to be unreliable for with basic business tools is a bit ridiculous. 

With load balancing off and all the office/outlook domains in its own rule with no scanning seems (so far) to have bandaided the issue. 

Could you post the domains you have in your firewall rule. 

This if for the Office streaming install as well as the outlook sync issue.

I am still getting the issue even with the rule created including the domains you have listed and few more that i have.  

what version?