Annelies,
Hi. SAP Work Manager 8.1 was released to support Maximo 7.6. This was also released in SAP Mobile Platform (Agentry pluggin version 7.X).
From July 31st 2015, the Agentry 6.0.X is end of extended maintenance (see: SAP note # 52505 for end of maintenance definition).
In order for you to work with Maximo 7.6 version please read and download the SAP Work Manager 8.1 for Maximo. This is available under service.sap.com <-- choose Support Portal <-- Choose Download software.
The issue could be that the libraries that you used in the older version 7.5.2 is not compatible with Maximo 7.6.
Hope this helps out. Please mark this SCN thread as answered if you find this helpful.
Have a great day.
Mark Pe
SAP Platinum Support Engineer