OK I finally found the problem. It was described inside an old thread in the old forum:
Help for newbie with quick setup of openhab
The problem was caused by the fact that the runtime folder was inside a directory where one folder contained a space in the folder name! That one directory is the ‘My Documents’ folder, which, to make things worse, Windows displays as ‘Documents’ most of the time (only when you click on the folder directory navigation space it reveals ‘Documents’ as ‘My Documents’…).
So Windows users remember to not put the runtime folder into a folder structure containing a space…!