Hello Marcus,
In version 7.x there was a separate External Captive Portal (ECP) license that needed to be applied for use.
At some point the customer must have enabled an ECP VNS for use and it will run for a short period of time before prompting the warning message.
Version 8.01 and newer includes the ECP support in the domain license.
Version 7.41 is over 5 years old, so their options would be to disable the ECP or follow an upgrade path to a current version 9 or 10.
This error should not impact other WLANs that are in use.
Regards,
Jason