If an FMU calls the logger callback provided by libcosim with a null category argument, the process will segfault. This is because of the following statement in src/cosim/fmi/v2/fmu.cpp:

BOOST_LOG_SEV(log::logger(), logLevel)
    << "[FMI status=" << statusName << ", category=" << category << "] "
    << msgBuffer.data();

Giving a null category is perfectly fine, and according to the FMI spec (version 2.0.5, section 2.1.5), it is the default thing to do:

If the FMU does not define any log categories, “category” must be “NULL”.

I haven't checked, but I suspect a similar issue exists for FMI 1.