I had to remove the solved tag, the error still persists. I have removed xf86-video-intel so KMS is used (i think so anyway?), but the error at startup is still there after the second reboot.
In the kernel log i found this post to;
[drm] intel_dp_start_link_train [i915]] *ERROR* failed to update link training
Must be something else than the old intel drivers that cause this? Any other logs i should look at?
-- Mike