I've just today started having problems with the web admin interface hanging on the initial load (ie when clicking from the bottom of the listinfo page, say.)

The only thing I can explain this by is the upgrade of a small web-based
mail program called Neomail (http://neomail.sourceforge.net/) which appears to have caused the problem. However, I can't see anything that
would cause this - they operate from completely different directories and neomail should not have altered any other config files.

The listinfo pages still seem to work fine.

The error I get is as follows:

[Wed Jan 31 13:38:57 2001] [error] [client 127.0.0.1] Premature end of
script headers: /home/mailman/cgi-bin/admin

Since you say there aren't errors in mailman's log, I assume this is in the web server's log? Aren't both Neomail and Mailman heavily assisted by the web server? Are you *sure* Neomail didn't change something about the web server configuration, like maybe something to do with CGI scripts?

and then on subsequent loads the page just hangs, and no further errors
are generated.

Doubtless because there's a lock file around from the CGI script exiting prematurely.