PD4ML stores some image (for example checkbox shapes) and configuration resources in pd4ml.jar and loads them using Java class loader.

Lotus database signing restricts some Java permissions and obviously an access to the classloader is among them. Unfortunately I cannot give you a hint what exactly needs to be (re-)configured by signing. But hopefully the direction where to look for the problem solution is more-less clear: PD4ML needs permissions to load resources with the class loader.