On my Imac pro with OS 10.14.5, I have been using both my internal Radeon Vega 64 graphics card AND a second, external GPU (Radeon Vega 64 in Sonnet eGFX box via thunderbolt3). In Preferences>System I have been using OpenCL mode, and selecting both cards.
This has been working really well.

But I just updated to 51, and unfortunately now, selecting the eGPU as well as the internal one results in problems. In the Edit tab, I select any video clip in the bin and no video appears in the playback pane. The playhead moves if i click play, and the playhead's black bar area fills ahead with grey, indicating cached video - but no video visible above this, the pane remains blank.

If i deselect the eGPU and restart, it works again (but without the extra power of the eGPU).

I know about the RCX51 support for nVidia for windows, did the changes made in that regard perhaps 'break' the support for openCL eGPUs...?

Please Help!