Thanks for the tip Heather.
Up until now LRF conversion worked fine through Calibre.
Got the book in question in LRF now by using a ZIP(!) version I also had
I'll definately get Lib2Go as an extra tool.
Edit: on the Lib3go-site it says: Lib2Go is based mainly on "calibre", a e-book library management application and utilites. Calibre is free, open source and cross-platform. You can download it here.
So probably the same "engine". I'll stay with Calibre, it always worked well in the past so it might have been something with the bookfiles.