yeah, maya is having a bit of a coniption,
try to find any node that isnt a default maya node and delete it, then it'll let you save as an ascii, also the vector render globals and or any vector render nodes seem to do this in maya 5
to get to the nodes, in the outliner click display>dag nodes only (toggle it to off) then scroll down to the very bottom, thats usually where youll find the offending node/s
fyi: it should be kewl if you select the objects u want saved and export selcted as an ascii, for an easier route.