I just installed Mplayer last night.
I had the same problem.
The solution is to :::
goto your preferences
click on the display tab
select a different display type
(you should heve 3 lines of display types. 1st is wide, 2nd is square and 3rd dunno. it did not work for me and gave the same error.
I just changed it to one of the other display types.
it all works fine