Error external mode on beaglebone

I am trying to run a model on my beaglebone black and sometimes the code runs, but sometimes appears:

Error occurred while executing External Mode MEX-file 'ext_comm':

Failed to connect to the target. Possible reasons for the failure:

a) The target is not switched on.

b) The target is not connected to your host machine.

c) The application for the model is not running on the target.

You might have clicked the Stop button. If the Run button is not dimmed,

click it. Otherwise, click the Build button,

which downloads and runs your application on the target.

Caused by:

An error occurred attempting to open an rtIOStream.

More detail may be reported in the MATLAB command window

I don't know what is creating this problem.