Alwayson Error

Hi , I can get alwayson timeout continuously.
Error Message
A connection timeout has occurred on a previously established connection to availability replica 'Server1' with id[XXXXXXXXXXXXXXX] . Either a networking or a firewall issue exists or the availability replica has transitioned to the resolving role.
Please explain this error and give an advice to avoid this in future.
Note: NT Authority/system, automatically remove it's sysadmin privileges.