edward
You could try this in a Terminal window:cd /opt/hfs19.5 # or wherever you installed houdini source houdini_setup houdini -foreground
Do you get the same OpenCL errors as OP in the terminal?
I able to fix by installing amd graphics drivers from the following link https://support.zivid.com/en/latest/getting-started/software-installation/gpu/install-opencl-drivers-ubuntu.html [support.zivid.com]