@Kaspar Kuldkepp V0.2.8 and all previous versions used a special part of blender's cycles engine to query render devices. In 2.91 the specific part we used changed, meaning we had to redesign how our addon uses that function.
This means that our addon will not load with Blender 2.91 with V0.2.8 or previous versions. You can still use 2.90, 2.83 LTS versions however.
This new build is being made ready for release over the next week or so.
For anyone else viewing this question, there will be a new version V0.2.9 being released to make Crowdrender compatible with blender 2.91.