Thanks MrSteveVee! I've been thinking. Maybe it has something to do with how DT has been installed. DT 2.6 is installed through the package manager (and therefore well integrated with the OS) but DT 3.0.1 is a Snap application. I guess it's not possible to activate opencl with the Snap DT. I assume I have to wait and see if I can get opencl to work when I can install DT 3 the proper way, from a repository.