It is related to Active Sensing as I have an unrelease hotfix from CopperLAN with Active Sensing disabled, it sound fine.
A while is around 2-3 mins of MIDI songs playback
I did connected through VMidi 2 port.
Turn verbose on and set midi.winmidi.device to VMidi 2, the error keep printed at every 500ms with synth\fluid_synth.c: line 1224: assertion `len > 0' failed
That is with Active Sensing enabled or using version CopperLan 1.2, I won't find this error with Active Sensing off or using version CopperLan 1.1
In CopperLAN Manager, I have linked VMidi 1 as input to VMidi 2 as output, I assumed you know how to configure it.