Hello Philippe,
> The situation has changed. I no longer get the Authentication page I described. But I still get the same error page.
Hmm indeed, there is an issue: the sync script between lists.spi-inc.org and archives-private.spi-inc.org only works if you created an account on lists.spi-inc.org (which happens on first login to it after the consent form you saw). I'll work on a solution. As a workaround, you can log in to lists.spi-inc.org and wait 5 minutes for the sync script to trigger.
Thanks for reporting!
> By the way, authentication now uses a "community account". Does this mean the passwords previously (as of 2021) used to access spi-private are obsolete?
Yes. There is just one password for all SPI-related services now: the one used on members.spi-inc.org.
Regards,
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Célestin Matte, IT contractor for SPI