Hi
whever people try to post comments to the site, they suddenly end up with the error message:

'Can't use an undefined value as an ARRAY reference at /web/script/latinowww/nowarblog.org/mt/extlib/jayallen/Blacklist.pm line 2819.'

Anyone any idea what this means? What we can do about it?

All help much appreciated.

That looks to be a problem with MT Blacklist, not Movable Type itself, so you might want to visit the MT Blacklist user forum for assistance with the error message.