All was solved by simply switching the RSS option on SMF off and then on again…
Correct, I fixed it in the meantime.
Turned out the Errors were due to the fact that I did not gave these specific pages a title in WordPress and that was enough for Twitter Cards not to be able to be produced. Fixed it by installing a plugin that removes titles, so I can still put in a title for the sake of Twitter Cards, but not show it on specific pages.
I followed your advice and used other RSS feeds in my wordpress RSS plugins, and they all work fine. I also tried the RSS feed of other SMF forums and they work fine too.
The issue you mention about not being able to reach my forum, I looked into. Turns out when I use the URL including www. those URLS don’t work for large parts of the day. The URL without www. always works. So I changed all URL’s to the version without www. for now.
These two issues might have started occuring at the same time, so might be related? I tried putting in the RSS feed URL with and without http://www., but that did not change anything.
