After long and hard search we came down to the following: The clients and the development machines seem to have different versions of the Microsoft agcore.dll (development machines have 2 different version in different paths). As this is the core of the exception we suppose this is the reason for the different error message - we were unable to fix this issue however (we cannot be sure what dll our clients get during SL5 download...) therefore we decided to rewrite the whole code segment to not make use of the exception message text at all. Lesson learned.
Thanks for the feedback. -Steve