Sorry for the long delay answering but I get no notifications from the OpenSprinkler Forums when someone answers.
My problem was caused by setting port 8080 like in the default URL in the Pebble app. I just deleted the :8080 at the end and it works now.
Too sad, Pebble development ended after Fitbit bought them. I hope Apple will improve battery life cause this is the only thing I don’t like on the Apple Watch.