UV unwrap creates a weird single edge result

I have a mesh which was decimated and UV unwrapped in zbrush, brought into blender to pack and optimize the UV’s. Turns out, one of the islands is messed up and streched. (Sometimes this happens with zbrush meshes, normally I just select the problematic island and unwrap it again in blender, normally works well)

this is what came out of zbrush, streched and bad:

When I try to unwrap it again in blender, this is what i get, super weird, only 2 edges?:

I’m out of solutions at this points and can’t figure out what goes wrong here. I’ve tried removing doubles or marking the problematic island’s border as a seam again and unwrapping, but nope.
Any suggestions are appreciated
Cheers