[otrs] SLA combo box contains only a dash

Hi there, I have enabled Service/SLA's and added a (valid) SLA and service. As a result, every ticket now has two new combo boxes in the free fields part: Service and SLA. But somehow the SLA combo doesn't display any values, it just contains a dash. The service combo box does work. Does anyone have any clues or hints to figure this one out? Thanks a lot, Stijn