Some time ago my PHPBB forum got filled with spam. I managed to remove the spammy content but Google still knew the links to these pages. When Google came back to spider these pages, my forum didn’t manage to explain that this content was actually gone.
Searching on the internet what was going on, today I found that not providing missing content messages with an HTTP 404 error message was a know bug for PHPBB 3.0.x. Fortunately it was solved with version 3.0.8 which I installed 2 days ago…
So now it’s just waiting for the Google spider to drop by and have it find out that the removed content indeed is gone.
Update December 4th: 4 days later the number of soft 404 errors decreased to 264. So it’s working!