There are several ways (functions provided by Maehara-san):

(1) Message lists similar to old ones:

http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/msglist
http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/msglist
etc.

(2) Old type format URL:
like
http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8965

This redirect wasn't working before. We had been trying to use those links without success. If they will keep working in the future, then we are okay ;)
I noticed that my message ended up as an attachment, I guess that only plain text messages are then displayed in the usual way and if there is some html code in a message, then it will end up as an attachment?

(3) Old messages
as before
http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8500

but until some
days ago I noticed that the old archives up to August 2021 were
still online, but now they aren't anymore.

   Something may have been changed.  Meahara-san, could you
check what happened some days ago?

Everything is working now, also the old links.
I guess there was something broken at some point.
Thanks for clearing up things.

Cheers,
Sebastian