Apparently Gnus is creating charsets. Since you use x-symbol, which
also creates charsets, I suspect you're running out -- the number of
private charsets that can be created is limited (10 or less). This may
happen in 21.5 rather than 21.4 because Unicode JIT charsets (new in
21.5) are using up some charsets.
If you're still dealing with this problem, you should try sending from a
Gnus started from xemacs -vanilla, without using x-symbol.