Following kernel seems to fix the issue (running on 18.10 with kernel installed by UKUU):
4.20.0-042000-generic #201812232030 SMP Mon Dec 24 01:32:58 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
I still see one or two of these errors when hotplugging but the others are gone and the external screen works:
[drm:intel_mst_pre_enable_dp [i915]] *ERROR* failed to allocate vcpi
Following kernel seems to fix the issue (running on 18.10 with kernel installed by UKUU):
4.20.0- 042000- generic #201812232030 SMP Mon Dec 24 01:32:58 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
I still see one or two of these errors when hotplugging but the others are gone and the external screen works:
[drm:intel_ mst_pre_ enable_ dp [i915]] *ERROR* failed to allocate vcpi