A strange thing just happened.
Before trying to export a model to an STL file I did SelOpenPolysrf. The model lit up. So I did Analyze > Edge Tools > Show Edges. No naked edges showed up.
I then exported the model to STL, and everything went fine. Usually when the are naked edges, I will get an window with an error message telling me that the model is no suited to exported for prototyping.
After having created the STL file I was able to print the model on my 3D printer without any issues.
Why would SelOpenPolysrf show the my object is open what apparently it is not?
I seem to be having many more issues with the current version (5A772w) than with several previous versions.