I have the following alert error that keeps popping up on my dashboard

I closed it yesterday, but this morning it is back. However, there are some strange things about it:

• 1n-ago : what does this mean?
• Full History : looking at the history it seems like it has been active continually for more than a month:
  
• We did have backend instability problems a month ago, but these have been sorted out. Looking at the data for the last 24h, I see none of the usual error indications. From these dashboards one can see:
  • no HBase NSREs,
  • Number op OpenTSDB connections seems reasonable
  • Bosun's open file descriptors and go routine count look stable
• Testing the alert in the rule Editor works fine.

How can I debug this further?
We are using Bosun 0.1.