here's an update:

i found a workaround which doesn't really solve the problem, but allows to at least save the file into the poser library after all groups and bones have been fixed by renaming them. simply save the model as a scene. for some reason, if you save it as a figure, it won't work properly anymore, but if you save it as a scene instead, it works. just remember to remove all unwanted items (such as lights, skydomes and other stuff)

i contacted poser's support and the first few days they tried to help me, but the support guy probably thinks i'm exporting it the wrong way in blender, so i didn't get any response for the last 2 weeks. he thinks the problem lies within poser's inability to handle FBX files above ver. 2014, which you can't adjust in blender.

PS: poser messes up it's own .FBX files, try it yourself. just export any rigged model (for example "andy") as an .FBX and reimport it, open the grouping tool or check through the setup room, you'll notice all groups and bones got some numbers added to them. you won't be able to mirror any poses anymore.