error – FBA just stopped working – SharePoint 2019 – where to start troubleshooting

Hello All ~ i recently inherited a SP farm, it uses HNSC on top of one webApp, so no Central Admin GUI / web-interface 🙁

FBA has been working for months, then ~2 weeks ago, users get message ”
The server could not sign you in. Make sure your user name and password are correct, and then try again.” EVEN when their user/pass IS correct.

there are 2 app servers, 2 NLB web front end servers, 2 NLB OOS servers, 2 dist cache and 2 search servers.

i believe the visigo FBA pack solution has been installed, but this really doesn’t matter as everything was perfect for many months until just 2 wks ago.

FBA does have a dedicated SQL DB on our SQL always on avail. group (instance)

i don’t see a dedicated FBA website or application pool.
the role provider and connection string are defined (local) on both web front end servers at the server level

i have rebooted both app servers & both WFE servers but nothing changed.
i used the ULS viewer on the WFE servers to view logs at : C:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions16LOGS

BUT saw NO events logged WHILE i was trying to login with an FBA account.
by the way everything else around FBA works (creating new users, etc…)

again this environment is new to me, and i’m more used to working in the CA web interface rather than strictly powershell

ANY IDEAS where i should begin to troubleshoot