Hi folks - we have had a lot of grief the last few weeks with our SEF extension. We tried to get it working with a new extension which badly affected other extensions.
Good news - we decided to replace it so if you've noticed problems these should now be resolved.
Bad news - moving to a new solution is painful and one of the side effects is some URLs will be broken. This will mostly affect the URLs in blog notification emails. If you have posted or followed a post and you have old email notifications in your inbox, clicking a link in those emails will result in a page not found error. Sorry - you'll just have to manually find your way to the appropriate post. You might find the options in the right sidebar handy (e.g. "My Discussions".
Just goes to show that it's not just you that has challenges with websites.