I just added a new feed using the browser but now when I try to open feed I get 'unknown error'. Now none of my homescreen apps work.
None of your homescreen apps, or just the RSS feeds?
There's another thread detailing the same sort of issue on the N8. The solution was to remove the "fring" chat app, and it worked on that device; resolved the issue.
Do you have "fring" installed on your N97m? If so, try removing it.
I'm not sure if the N97m has any issues with hard reset permanently losing things but, if not, why not backup, hard reset, restore, see if that undoes the bug.
I presume you deleted the rss feed you added already?