It appears the underlying issue is someone has changed Mailman's character set for 'nl' (Dutch) from iso-8859-1 to utf-8. Possibly whoever did this did the appropriate things such as recoding the message catalog and templates to utf-8, but in any case, the strings in the attributes of this list weren't recoded. This is one of the major problems that make it difficult to change Mailman's encoding for a language. See the definitions of the recode(), doitem() and convert() functions in Mailman/versions.py in Mailman 2.1.19 or later.
So basically, this issue appears to be a 'shot oneself in the foot' thing and probably could be fixed by setting the list's description to 'Lijst voor Caljent\xc3\xa9-leden', although I would be concerned that there are other iso-8859-1 strings in list attributes.
Anyway, I see this as an issue worth fixing. The fix I would propose is in Mailman/Handlers/CookHeaders.py replace the line at the end of the definition of uheader which is currently
return Header(s, charset, maxlinelen, header_name, continuation_ws)
try: return Header(s, charset, maxlinelen, header_name, continuation_ws) except UnicodeError: syslog('error', 'list: %s: can't decode "%s" as %s', mlist.internal_name(), s, charset) return Header('', charset, maxlinelen, header_name, continuation_ws)
** Changed in: mailman Importance: Medium => Low
** Changed in: mailman Status: Incomplete => In Progress