I've looked through the osm data (in JOSM) but found no irregularities. All nodes are connected, there's no oneway in the wrong direction or anything. The ramps are tagged with ref=A4;B4 which seems odd, but I don't know the proper handling of ramps regarding ref numbers.
Motorway and motorway_link are implicitely oneway. This is correct according to the OSM wiki and the style rules of mkgmap reflect this.
In your example there is a part of the ramp which is common for both directions. This part must be set "oneway=no". I made that change a minute ago. Please try again.
Thanks very much! I never mapped a motorway_link myself, so this was new to me. I can't try it now, 'cause I don' have a Windows machine until tomorrow afternoon, but I suppose it's working now. regards Sefan