On some computers CASA starts with an "Xvfb" error message. The "X virtual frame buffer" is only used by the viewer to transfer non-GUI outputs to the terminal. For most applications this is not really needed. But for a permanent fix, one may install xvfb on the system. E.g. via
sudo apt-get install xvfb
followed by a restart of CASA.
If you still receive the errors after installing xvfb, it usually is not a something to worry about as long as you can see the logger, viewer, etc and if you can communicate with those windows.