sendCustomScript() disconnects with UR Cap

I'm trying to use the OnRobot RG2 gripper with the ur_rtde library. Therefore I followed the example, by splitting the rtde_control.script into init and control. I'm using the FLAG_USE_EXT_UR_CAP to create the controller. Then I try to execute the function rg_grip(20,20) with the sendCustomScript(), but every time the connection is terminated. Is there a reason for that? Are there otherways to use UR CAPS with this library?