Weird error in log when using Thin Server with a pretty simple rails application

I am pretty new at using the Thin server on my simple rails application.
It's working nicely except, that 1 out of 4 times more or less when I go
to any page in the app I get this on the browser:

    Status: 500 Internal Server Error Content-Type: text/html
    Application error

    Change this error message for exceptions thrown outside of an action
(like in Dispatcher setups or broken Ruby code) in

and this in the log:

Before I continue to kill myself troubleshooting this, I wonder if
anyone has seen this and can help me with a clue or two? Thanks!!