If you know that the QLA Server lists the correct serial number, you will need to obtain the correct validation code for that serial number and reinstall QuarkXPress. If the QLA Server lists an incorrect serial number (this is a less likely scenario), you will need to download the correct QLALicense.dat file that will match your site serial number.

Background:
This error occurs because the serial number for the copy of QuarkXPress you launched does not match the serial number that is supported in your QLA Server. This is the only reason this error can occur.