I have had this problem on several occasions. When I edit .py-files of existing modules (yeah, I know this is considered bad practice, but that is another question), e.g. add a new field, I get the "no handler found" error when I try to access through the  browser client after restarting the server. If I comment out my additions and restart, everything works fine again. I simply cannot see where I go wrong, as I have succesfully made additions like these in the past (and the syntax etc. checks out).