Hi
You can see __404__ article correctly activated directly in the screenshot.
How about the category?
Activated "Sh404sef Multi Language Compatibility" in JSitemap PRO Plugin
What the heck is that???
OK, I can at least imagine that this is caused by JSitemap pro. Rather than spending a long time on this, we 're about to release a new version where the 404 page mechanism has been changed, precisely to avoid problems with extensions like JSItemap which also want to handle the 404 page.
Please download it from here: xxxx
Then install over current version (BTW, I assume you're using the latest version of sh404SEF?).
The __404__ article and the sh404SEF custom content category are not used anymore. The error page is now only modified from the sh404SEF configuration ("Error page" tab). If you have customized the __404__ article, the changes you made will be copied over to sh404SEF.
The old __404__ articles and the category are left on your site, but unpublished as they are not needed anymore.
Please let me know.
Rgds