Frequent logging out of users in Sophos XG Firewall

We are facing a problem of users getting logged out quite often and reopening of the captive portal again and again in a very short interval. I will be describing my scenario in detail below

• We have ~9000 users in our system
• We have 2 XG 750s in Active-Passive HA configuration
• I have tried the scenario for a user with:
  • No filtering policies
  • No login restrictions in terms of number of simultaneous logins or IP restrictions
  • Not an administrator user

For some log, I have tried using Sophos Client for Linux (CAA) and whenever I get disconnected, I get the message in the client as :
You have been disconnected by the Administrator.
and the client even gets terminated!

We have even seen the same message in the Network Agent app for Android.

Point being

• I am the administrator and I haven't disconnected anybody.
• We have not made any changes to the firewall when the message occurs.

I would like to know what all are the scenarios when this message can come (obviously other than when the Administrator actually disconnects) so that we can narrow down the problem.

We have been facing this problem on both wired (~40000 LAN points) and wireless (~800 access points [not by Sophos])

Regards,