Unable to connect - Too many sessions are already opened

[Humanbeing]

Dear Community Members:

I have two Registrar Modules.  The Domain Synchronisation Cron Report Email says that the first Registrar's domains are "in sync."
But for domains from the second Registrar, it gives error: "Unable to connect - Too many sessions are already opened." 

The second Registrar says their other clients' WHMCS are working normally.  
What could be the cause and fix for this: "Unable to connect - Too many sessions are already opened."

Thanks in advance!