Have you tried updating the iPads to the latest app version? I’m having the same dropouts here and quite certain it’s due to running the latest server version with older client software apps that’s causing the problem.
(Unfortunately for me, I can’t update my iPads to confirm if this is right. The latest app is now being listed by Apple as incompatible for an iPad 9.7″ 4th Gen running iOS 10.3.4)
Lonny