Error running in Simulink external mode with Arduino

I have been using an S-function I created in Simulink to communicate with multiple compass sensor modules for the past couple of months with no issue, but for whatever reason it stopped working this week and I started getting the error:

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

I am only getting this error when I run this particular s-function in external mode. The board works with other models including other s-functions I made with a very similar format) and this particular model is able to be deployed to hardware as well. Any idea why it stopped working?