File associations with windows - especially: I can't set the default program for an extension anymore

This might expand in the future.. windows registry is deep ;)

My problem was, that using either of those 2 dialogs didn't help to set the default program. I chose a file, but no association was made. Why???

Neither here: Control Panel\All Control Panel Items\Default Programs\Set Associations

Nor here: Open with dialog

In the end, this link gave me the tip:
fire up regedit, go to
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts
and then delete the "OpenWithProgids" subkey from your extension:

The id is probably broken, who knows?

In any case, seems like the id prevented me from assigning a different default program. Once that's gone, you can re-assign any program with the extension that you want.