I might have found the issue and it seems to be linked to KLM only serving GLA and KLM only offering HKG 4 days per week from AMS.
If I select GLA-AMS-CDG-HKG outbound and HKG-AMS-CDG-GLA inbound on a day when KLM have a HKG-AMS on the return then I can book premium comfort. If it’s on a day where KLM don’t have a HKG-AMS flight then I get the error.
Therefore I just need to adjust my days slightly.
If only the website would have told you that ... big IT fail
