The only fault I know was in the modbus Slave library regarding coils, however that one has probably been fixed a while ago i guess, and is easy to fix yourself. The master library however works like a charm
If the problem persists, these could be some options?
1 - ignore that virtual com port software and connect directly. If that works long-term you know whats wrong
2 - If that not works, run a little bench test on the equipment. Speed up the communication and see how the instrument behaves. Try writing a lot of holding registers and see the handling of thoes, and the time delay before the instrument answers. Some instruments may react terrible after a while under pressure, and may have some buffer issues, or do not keep up but still acknowledge that the registers have been set, but internally it processes the data afterwards. giving these slower instruments some processing time might solve something. If this crashes before 2-3 days you might know the fault
3 - A work around if still stuck is re-establishing the communication when it times out. If that doesnt work its instrument related as you actually must reset the instrument?