Evertjan, your problem is not the problem discussed in this bug report. Your error log contains
D [13/Jul/2012:22:33:47 +0200] [Job 42] argv[5]="InputSlot=Auto number-up=1 PageSize=Custom.BrA3_S MediaType=Glossy noOptionDuplex OutputMode=Best ColorModel=RGB Duplex=None job-uuid=urn:uuid:f76c2ee9-31be-316e-7cb6-7764469f48f3 job-originating-host-name=localhost time-at-creation=1342211627 time-at-processing=1342211627"
This means that the application sends a broken PageSize attribute: "PageSize=Custom.BrA3_S". So your problem is bug 998156.
Evertjan, your problem is not the problem discussed in this bug report. Your error log contains
D [13/Jul/ 2012:22: 33:47 +0200] [Job 42] argv[5] ="InputSlot= Auto number-up=1 PageSize= Custom. BrA3_S MediaType=Glossy noOptionDuplex OutputMode=Best ColorModel=RGB Duplex=None job-uuid= urn:uuid: f76c2ee9- 31be-316e- 7cb6-7764469f48 f3 job-originating -host-name= localhost time-at- creation= 1342211627 time-at- processing= 1342211627"
This means that the application sends a broken PageSize attribute: "PageSize= Custom. BrA3_S" . So your problem is bug 998156.