A warlock player I know found a third way to fix this, maybe this will give some insight into the possible causes.
1. Ultravision ends on player characters, either by running out or resting and all other characters will appear as invisible to them, that is you can not see them until very close and you can not target them at all
2. What the player does is, they cast Darkness on our group, then cast darkness somewhere away from us, then dispell the darkness on the ground
3. This makes the bug disappear and we can see normally again