Did you try this?
SOGoMaximumPingInterval = 3540;
SOGoMaximumSyncInterval = 3540;
SOGoInternalSyncInterval = 30;This is mentioned in SOGo installation guide, it works for me. But i don't have as many clients as you.
If it still raises error, it's better to post in SOGo mailing list to get support from SOGo developers.
http://sogo.nu/support/index.html#/community