In my example I found that the path itself is the problem
Probably because it becomes part of the volume along the edge
Once I removed the path the volume was calculated as a whole.
I retried when moving the faces slightly away from the path (so not connected)
Somehow the volume was not calculated, but when I now deleted the intermediate faces as TIG suggested I got the complete volume,
Strange...
Anyway its working now, thanks for the suggestions!