How to fix "Error occurred while executing External Mode MEX-file 'ext_comm': ExtTargetPktPending() call failed while checking for target pkt" ?

Hi, I have a very simple simulink model. I want to read the signal at some analog and digital inputs and show them in a display and scope. I also put some PWM signals to two digital output pins. Additionally, I include a c-function to my model with a matlab function. So far, everything is working but if I want to reduce the step size in the model configurations from 0.001 to 0.0001 I get the following errors:

Error occurred while executing External Mode MEX-file 'ext_comm': ExtTargetPktPending() call failed while checking for target pkt

Error occurred while executing External Mode MEX-file 'ext_comm': ExtSetTargetPkt() call failed on CLOSE. Ensure target is still running

I am using a ARDUINO UNO Board and the build process can be completed successfully.

Does anybody know how to fix this errors?

Thanks for your answers!