Video stuck at 640x480 SLED 15 SP1GMC as VirtualBox guest

Testing SLED15 SP1 GMC as a VirtualBox guest (on a Leap 15.0 host) and video is stuck at 640x480, barely usable.
Tried building the VBox Guest Additions, but the video module didn’t build, complaining about “wrong config of kernel sources” or lack of autoconfig.h IIRC.
The other VBox kernel modules (e.g. for shared folders) were built fine, the usual set of kernel-devel etc. is installed.
Am I missing something obvious?

You need hardware acceleration for good multimedia support (like play a H.264 video).
There is no fine working hardware acceleration available for SUSE Linux Enterprise (SLE). For more information see: