Mailman error when visiting lists

I navigated to https://common-lisp.net/independent-lists and clicked on
small-cl-src.  This produces the following error:
Bug in Mailman version 2.1.23 We're sorry, we hit a bug!

Please inform the webmaster for this site of this problem. Printing of
traceback and other system information has been explicitly inhibited, but
the webmaster can find this information in the Mailman error logs.

I tried a few others and got the same error.

-- 
Ray