The most obvious thing to look for is a change in the environment. Has something in the setup changed due to a package update or a firewall change that has shortened the allowed session length for example?
The next possibility would be if there was a new message with extremely large attachment(s) that might be causing an out of memory issue for PHP
Have you seen any errors in the z-push-error.log file?
Do you see any sessions in the z-push.log file that do not end cleanly? For example, every Sync command should end with these 2 lines - one which shows the memory usage for the request and the second which just shows ------- End.
13/11/2017 10:52:36  [ INFO] [userId] [8f73c810d4154c398be40e7c955d7700] cmd=‘Sync’ memory=‘87.23 MiB/94.50 MiB’ time=‘9.92s’ devType=‘WindowsOutlook’ devId=‘8f73c810d4154c398be40e7c955d7748’ getUser=‘userId’ from=‘220.127.116.11’ idle=‘0s’ version=‘2.3.9beta0+0’ method=‘POST’ httpcode='200’
13/11/2017 10:52:36  [DEBUG] [userId] [8f73c810d4154c398be40e7c955d7700] -------- End
Have you checked the Apache error log(s) for issues?