PFSense Squid Guard proxy filter locking domain user accounts constantly

[sborg]

We have in place PFSense Squid proxy server, squid reverse proxy and squid guard proxy filter which filters internet for all users in the domain. All users have to authenticate themselves first in order to access any internet sites, a pop up appears in their internet browser asking for their domain user credentials or any app that requires internet to function. At the moment we have a around about 4 users which are constantly being locked out of their domain account. We switched off the authentication part of squid proxy in order to determine whether it is actually the proxy or not that is causing this issue and indeed it is the proxy server as when it was not enabled none of the users were locked once throughout the day.
Has anyone encountered such an issue before? And is there something I need to check on what is causing them to be constantly locked out?