Søren O'Neill wrote:
Hi Herbert and Axel - that was indeed the problem ...obviously that leaves me with a problem: Captions and table labels etc, which normally go inside the float ...
LyX is buggy in this area. The best way is to write the code of the longtable in LaTeX, save it as an own file and then inside LyX Insert->Include File->Input and enable preview. Longtable has its own counter and the caption must be part of the table itself. See doc of longtable.sty Herbert
