unable to connect with ActiveSync Client on Sophos v 18.0 MR 4

Hello Community,

i have a weird issue while trying to connect to Exchange (2013) with some mobile devices (android and iOS). When i take a look at the webfiltering everything seems fine:

I have a lot of devices that can connect flawless while new created devices don´t connect to ActiveSync at all. There seems to be no difference in the logs with systems that can and cannot connect:

Log from a iOS device () with Outlook App () with working App (send/receive Mails ok):

tail -f /log/reverseproxy.log | grep -i username

[Wed Jun  9 15:15:27.273917 2021] timestamp="1623244527" srcip="client_ip" localip="ext_ip" user="-" method="POST" statuscode="200" reason="-" extra="-" exceptions="SkipBlacklistDNSRBL, SkipBlacklistGeoIP, SkipAntiVirus, SkipURLHardening" duration="49530395" url="/Microsoft-Server-ActiveSync" server="mailserver" referer="-" cookie="-" set-cookie="ClientId=ZNQAVTFKGAQMAXECWZW; expires=Thu, 09-Jun-2022 13:15:27 GMT; path=/; HttpOnly, X-BackEndCookie=S-1-5-21-2403694064-3893359420-3755250045-10265=u56Lnp2ejJqBnp7Gns3HzMrSzsfNndLLmcaZ0p2dzcrSnMybm52enJnHm8+bgYHNz83O0s/I0s/Gq87Mxc7Jxc7J; expires=Fri, 09-Jul-2021 13:16:16 GMT; path=/Microsoft-Server-ActiveSync; secure; HttpOnly" recvbytes="487" sentbytes="729" protocol="HTTP/1.1" ctype="application/vnd.ms-sync.wbxml" uagent="Outlook-iOS-Android/1.0" querystring="?Cmd=Ping&User=tmb%5Cusername&DeviceId=c34f87eddcb44226a241bc34a42b67b9&DeviceType=Outlook" websocket_scheme="-" websocket_protocol="-" websocket_key="-" websocket_version="-" ruleid="35"

Log example from a iOS device () with Outlook App () with working App (send/receive Mails NOT ok):

tail -f /log/reverseproxy.log | grep -i username

[Wed Jun  9 14:05:07.943894 2021] timestamp="1623240307" srcip="client_ip" localip="ext_ip" user="-" method="POST" statuscode="200" reason="-" extra="-" exceptions="SkipBlacklistDNSRBL, SkipBlacklistGeoIP, SkipAntiVirus, SkipURLHardening" duration="20077554" url="/Microsoft-Server-ActiveSync" server="mailserver" referer="-" cookie="-" set-cookie="ClientId=AQFLUEBY0AQXZQSALHG; expires=Thu, 09-Jun-2022 12:05:07 GMT; path=/; HttpOnly, X-BackEndCookie=S-1-5-21-2403694064-3893359420-3755250045-14328=u56Lnp2ejJqBnp7Gns3HzMrSzsfNndLLmcaZ0p2dzcrSnMybm52enJnHm8+bgYHNz83O0s/I0s/Gq87Nxc/Kxc3I; expires=Fri, 09-Jul-2021 12:05:27 GMT; path=/Microsoft-Server-ActiveSync; secure; HttpOnly" recvbytes="542" sentbytes="731" protocol="HTTP/1.1" ctype="application/vnd.ms-sync.wbxml" uagent="Outlook-iOS-Android/1.0" querystring="?Cmd=Provision&User=tmb%5Cusername&DeviceId=893975bacf1f420c9f22592a65856709&DeviceType=Outlook" websocket_scheme="-" websocket_protocol="-" websocket_key="-" websocket_version="-" ruleid="35"

I can see no difference in these two logentries, but in the second case the user gets no mail nor can he send one.

Exchange Active Sync Checker (https://testconnectivity.microsoft.com/) shows everythink is setup well.

How can i find out what is going on?

Thanks for any hints.