You can definetly use the fast track as SK EBG+ (but not other *A FFP) when flying any *A.
I have recently been on TP, LH, SN and LX out of CPH without issues at fast track. Though the agent at the SN/LX counter was not sure.
May by there have been some issue with your BP? It happens.
The sign next to the fast track is on purpose misleading, one SAS employee told me – not to encourage other *A FF’s to use it.