Hello ... it's me again. 
My posts are still being moderated - though I want to keep you up to date.
Again I conferred with the customer who's facing that issue - they want to keep an eye on it but have no further transport requests to reproduce the problem at the moment - so maybe it was more of a user error than a technical problem - though I'll keep watching that issue and inform you as soon as there are any news.
Thanks so far for your helpful and quick responses.
Best wishes
Marie