What version of VIrtualBox do you use?
Do you have ran installation with restricted addons included (checked [x] Install third-party software for graphics and Wi-Fi hardware and additional media formats)?
It may be known Xorg/GStreamer bug, which is out of our responsibility.
Which application are you trying to start?
If it is Welcome and you have restricted addons enabled - apply method from other thread and then reboot VM.
Did you tried to reset mate-panel by doing right mouse click on it and selecting Reset Panel?