yes, installing xinerama fixed it, thank you!
(I am indeed using Ubuntu 16.04)
I will now install Runtime 19b. I don’t know if you are one of the site admins, but if yes, maybe you could change the information on the Download page, because it still says 2015b, in the installation instructions for BrainSuite21a, and also Brainsuite16 is mentioned.
Thanks a lot, once again!