Exchange 2016 Update to CU19 and latest security patch / SMC EAS Proxy not working anymore

Hi there,

yesterday evening we updated our Exchange 2016 with CU19 and the latest security updates. Now all SMC managed devices (iPhones) are not able to sync through the embedded EAS proxy.

EASACCESS log says

2021-03-05 11:05:05,422 INFO  [com.sophos.mobilecontrol.easproxy.easproxyservlet.accesslog.AccessLogger] (default task-23) user "BLAH" with active sync id "BLAH" has permission "TRAFFIC_ALLOWED"

EASPROXY log says:

2021-03-05 11:05:05,406 INFO  [com.sophos.mobilecontrol.easproxy.easproxyservlet.ProxyServlet] (default task-23) [BLAH] verify user 'BLAH' with active sync id 'BLAH', deviceType is 'iPhone', user-agent is 'Apple-iPhone9C3/1804.52', protocol version is '16.1', request method is 'POST'
2021-03-05 11:05:05,453 INFO  [com.sophos.mobilecontrol.easproxy.easproxyservlet.ProxyServlet] (default task-23) [BLAH] transferred 0 kB to exchange
2021-03-05 11:05:05,453 INFO  [com.sophos.mobilecontrol.easproxy.easproxyservlet.ProxyServlet] (default task-23) [BLAH] received response code from exchange is 413
2021-03-05 11:05:05,453 INFO  [com.sophos.mobilecontrol.easproxy.easproxyservlet.ProxyServlet] (default task-23) received from exchange header 'Server' = 'Microsoft-IIS/10.0' and header 'MS-Server-ActiveSync' = 'null'

ActivesyncID and USER anonymized

Any ideas?

Regards
Chris