Hi Sebastian,
I'm afraid that you might use the utility in a wrong way so you're faced with the issue.
The most likely cause would be that you've done
require_once of many classes including those from services, e.g.,
CampaignService.php, which in turn includes
ApiError.
Thus, when you call the deserializer, it crashes because it can't find such a member variable.
Please see similar case
here.
We're happy to help in this forum, but we would appreciate more if you could ask library-related questions on GitHub, so other users can see as well. :-)
Please let me know if anything is still not clear.
Best,
Thanet, AdWords API Team