Hi,
OpenCL works fine otherwise, and I used gr-fosphor
successfully on this same machine before.
Which version did you use and with which drivers ?
I am on Ubuntu 18.04.03 LTS with the proprietary
Nvidia driver (v390). The gnuradio is 3.7.13.5 (comes with the ubuntu repo, not compiled
from source).
Try reverting a269271fed026d5e633fa45f4dd5db125133f453 and see if that helps.
Cheers,
Sylvain