multiFEED uses the BlackBerry 10 Hub to report any problems it encounters while downloading feeds. The feed URL it was attempting to load is listed in the Hub alert, which should point you to the affected feed so you can troubleshoot it.
Unless you have incorrectly typed or copied the feed XML URL most errors are temporary and can be ignored as multiFEED will try to load the feed again after the next refresh interval has elapsed. Alternately you can try to load the feed again immediately with a "pull-to-refresh" on the Front Page. Sometimes you may get many error alerts in a row, such as when you lose WiFi or mobile network connectivity. Again, you can usually ignore these warnings as multiFEED will try them all again after each feed's next update interval.
Occasionally feed load errors may be due to temporary feed server down times. Once again, multiFEED will continue to try the download from the server after each refresh interval.
If an error repeats after every refresh interval with no sign of resolving itself, then either you have entered the feed URL incorrectly, or the feed publisher has removed the feed or changed the URL (which does happen without warning sometimes). In either case, use the Test Feed URL feature on the Feed Settings or Subfeed Settings page to diagnose the problem. Also, you can go back to the feed subscription page if needed to see if the URL has been changed.
If you don't want multiFEED to report errors in the HUB you can turn it off. Alternately, you can override the global setting for each feed or subfeed individually. This can be useful in cases where you want to see errors in the Hub, but one or more of your feeds are intermittent and generate too many messages. Simply enable Hub error reporting globally, and then disable it explicitly for the inconsistent feeds and subfeeds.
If you just can't resolve the cause of feed download errors contact arsMOBILIS Support for assistance.