[AUCTeX] Folding: some maths characters are not displayed

[Frederik]

Hi there!

I've got problems when folding some maths characters.
Most of the symbols display correctly, if I fold the buffer, but some 
are just displayed as a single empty box.
If I check for example the "Latex Fold Math Spec List", epsilon, amalg, 
preceq, succeq, Join, many arrows and other ones are not shown correctly.

I remember having this problem worse, but after an update of 
Emacs/AUCTeX many symbols appeared, others stayed as a box.

It seems not to be an AUCTeX specific problem, because if I copy one of 
the boxes, which "represents" a symbol, and paste it e.g. into my 
Thunderbird text field, the symbol appears correctly.

What could cause the problem?

Thanks and regards,

Fred