@jvstratum this was already set in SH404SEF. That setting had been my first thought (see my reply from 11 May). The code example in my 5 June reply shows the URL in the email template, which means ACY is either already processing the URL through SEF logic or that something else is conflicting with the URL generation process. In either case, it seems unlikely to be an SH404SEF issue.
@lifeintravel I worked around the issue with a couple of .htaccess rules that intercept the incorrect SEF URL and rewrite it back to a non-SEF address. I hate to have to do this, but I'm unable to provide backend or FTP access to this site. Keep in mind that this solution worked for me (and I will have to maintain it if the extension changes), and you will have to adapt it to your specific site/setup. Don't forget to replace example.com with your website address:
#Browser view
RewriteCond %{QUERY_STRING} userid=(.*)
RewriteRule ^component\/acym\/archive\/([0-9]{1,5})-(.*)$ https://www.example.com/index.php?option=com_acym&ctrl=archive&task=view&id=$1&tmpl=component&userid=%1 [R=301,L]
#Unsubscribe links
RewriteCond %{QUERY_STRING} id=([0-9]+)&key=(.*)&tmpl=component
RewriteRule ^component\/acym\/frontusers\/unsubscribe\/(.*)$ https://www.example.com/index.php?option=com_acym&ctrl=frontusers&task=unsubscribe&id=%1&key=%2&tmpl=component [R=301,L]
I'd still be thrilled to not have to use this workaround, so if anyone is able to figure this out, please reply here so we can all benefit!