Issue is for the admin user only... debug 2000 is inconclusive
I manage to trick it to login
I use reseller user to command users so I login as one of resellers from whmcs - that opens ok , then I login as admin from whmcs admin login ... that got the error but first window change user to admin and I can switch from reseller user level to admin user level in that window and see the panel\
theme change do not affect issue
It is getting late in my current location I'll look at it for some time and if not fix it will try tomorrow to open a ticket