That error means it got a web response but the content of the web response was not a feed. A web page url entered as a feed url would do that or if the feed url was throwing an error page instead of a feed, but in that case there should be a separate error logged from the forum feedurl in addition to the one logged by feed manager. I would check the feed again and check the url for the feed in feed manager.

Actually looking at the feed url you posted it doesn't look like the correct format for the latest version of mojoportal, I would copy it again from the feed icon link in the forum. In the new format we have combined the module id and item id into one ~ separated parameter parameter like this:

/Forums/RSS.aspx?pageid=5&m=34~2

another thing you could try if the forum is on the same site as the feed, a relative url in the format ~/Forums/RSS.aspx... might work there.

Thanks for the beers! Much appreciated.