Devices showing offline suddenly

You can probably save yourself some time in the future if you check the first bug reports in the community – created wiki.

http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports

In this case, there have been two separate issues with the same symptoms reported in April. One started about three weeks ago, so it may not be related to the one you saw.

However, the other just started on April 20. a hot fix was just released for it today, but some people say they are still seeing the problem.