There may be a problem on either end of the channel. There is no strict specification of the SLCAN tunneling protocol, so every implementer has to make their own assumptions, thus possibly disrupting cross-vendor compatibility. Consider connecting to your device via a simple serial line terminal emulation program (e.g. PuTTY works well on Windows) to investigate what is going wrong during the initialization sequence. The implementation of the SLCAN driver used in the GUI tool can be found in the Pyuavcan repo. The initialization part is specifically here:
P.S. I personally found the hardware from VSCOM to misbehave frequently and to be generally unreliable, but it may have no relation to your current issues.