Hi Alf,
I have no experience with the dongle or ARCP. In fact, I found ARCP to be less than useful and use other specific software for digital modes. I did discover the following, which may help:
1. WSJT-X - put radio in VFO mode for FT8 (upper sideband, mic gain=12, power=30)
2. MixW2 - used for PSK/RTTY (upper sideband, mic gain 12, power 30/100, respectively)
3. Try FlDigi - this is working for PSK, RTTY and even CW
I am able to run all the digi-modes using the above software and settings. Also, the "order" in which you launch the software effects which one gets the USB assignment in Windows. This might be your issue.
Lastly, you may need to go into the Device Manager (Start Menu/Computer (Win7) or search in Win10), find the assigned Com port, double-click it and verify baud, stop/start bits have been assigned correctly for the connected device.
I hope I've helped and not confused you.
73, Dennis - WU6X