KMail is irretrievably broken

[René J.V. Bertin]

You have your archive in maildir format on a server, no? I can't remember how you accessed those archives, but if not via NFS or other remove filesystem you might try that (in case you didn't already try to import the stuff from local maildirs, of course).

The other thing to try involves replacing your archive format with IMAP and see if that works better. That might be as simple as installing an IMAP server which understands maildir folders, otherwise you'd have to use a simple no-frills MUA that understands both formats to copy everything over.

I kept an email archive that way for years, still do in fact (but only access it on the machine which actually hosts it because it's not always on).

HtH,
R.