you seem to be an experienced integrator and do not know the obvious things.
you or your client pressed the kick everyone button in the menu (you have now hidden it)
in the description of the button it says "This menu item is displayed only for contacts with the access level "Administrator". When this setting is enabled, all employees with the access level "Employee" are automatically thrown out and will not be able to log back into the system until this setting is turned off"
Turn on the button - turn off its action and you will be happy
good luck
I spent a lot of time to find what you slammed
if you want to drop a tip here is the link)
https://www.liqpay.ua/ru/checkout/card/i40375101891