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.

================================================================
Dear Bug Team!

This is bizzare: I have install a no-gnus version which I
use either with 21.4.22 Or 21.5.29.

I wanted to send a message with 2 PDF attachments. It worked
fine in 21.4.22 but in 21.5.29 I obtained an error which I
attach. Since it works in 21.4.22 I hesitate to send a bug
report to the gnus list.

Uwe Brauer