The first look at your zip file shows that the problem arose because the file openmeeg_hm.mat is empty. Thus the error is not in the om_minverser but in the previous stage that generated this file. om_assemble.
Looking at the log file openmeeg_log.txt, it clearly says that two of your meshes are self-intersecting... This is probably the reason for which om_assemble produced and empty file. I consider this as a bug. The warning should be a clear and reported error. I will make a change to improve that, On the other hand, this means that openmeeg is not faulty here and that the tool that you used to produce the meshes did not work properly.
The log also shows some strange output for Interfaces "^H^H" as second parameters. I will have a look on that too. But I do not think it can have a link to the fact that meshes are self-intersecting.