Alrighty Then!

After a crash my eclipse did not longer know how to edit .mxml- or .as-files:

Unable to create this part due to an internal error. Reason for the failure: The editor class could not be instantiated. This usually indicates that the editor’s class name was mistyped in plugin.xml.

The easiest solution i found is on:
http://www.machine501.com/2007/07/05/flexbuilder-wont-start/

In short:

• quit eclipse.
• go to [your_workspace_dir]/.metadata/.plugins/com.adobe.flexbuilder.codemodel/searchCache and delete the directories (not the file “catalog.xml”).
• restart eclipse.

Making a backup might be a good idea though.