The problematic entry is Hierarchical Subject.
For reasons I do not yet understand, APG 'translates' certain keywords/keyword structures to 'empty', and LR4 cannot deal with this. My current working guess is that LR 'sometimes' puts Hierarchical Subject
(=keywords) into the image in the form Child | Parent | Grandparent
and APG converts that wrongly somehow.
All my non-importable panos (that I have checked so far) show more or less this in exiftool -q -H -G -U -a pano.jpg |grep XMP
[XMP] - Subject : , , , , ,
[XMP] - Hierarchical Subject : , ,
If I delete just the Hierarchical Subject entry using this command:
exiftool -XMP:HierarchicalSubject= file.jpg -o fileNew.jpg
the newPano.jpg imports into LR4 without any problem.
I am not sure, when I will find the time to analyse which keyword input produces the problematic subject.
Maybe someone from APP can have a look at the hierarchical-keyword generation if there is an exception case causing what looks to me like a list of empty elements.
Greetings and thank you to everyone for the time and suggestions