Unable to save token - MM + Gitlab SSO

Troubleshooting
3
  1. No, Standalone Mattermost
  2. This is occurring on 5.18.1 - before and after upgrade
  3. No changes from my side, to replicate, I just try and login.

System Design:
Gitlab in Datacentre, Mattermost hosting in AWS, SSO call is a remote call to Gitlab. As noted this has been working fine for months. However, I restored Mattermost database to an instance inside Datacentre, no there changes made, except for database connection string, and working as expected.
It appears to me, that the token call is time-sensitive/latency, albeit around 1 sec at worst.

Same Gitlab used in DC and from AWS, same account, same version, same database. Only difference being location and distance.