Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.
Please note, we don’t officially support macOS with our DisplayLink-based docking stations (like your UD-ULTC4K) and USB graphics adapters, as there have been historical driver incompatibilities in the past. (We do mention this on our website and product listing, but realize it can be easy to miss.) That being said, there has been significant progress made by DisplayLink with macOS Catalina and Big Sur with an updated driver and most of our customers report good results with this setup, and if this trend continues we hope to change macOS to a supported platform.
With that said, there are known issues with DisplayLink based docking stations and graphics adapters at this time when being used with clamshell mode, so this mode is not currently supported. This mode may be corrected later with future DisplayLink driver updates, but for now we recommend to avoid closing the lid while using our dock.
Please don’t hesitate to let us know of other questions.
Thanks again for contacting Plugable support and best wishes!
Senior Engineer | Product Owner