If an existing working- from the command line - Roo-project for whatever reason does not have the file "com.springsource.sts.roo.core.prefs" in ".settings" STS 2.6.0 will trigger the erroneous message "No valid Spring Roo installation configured. Use the "Roo Support" preference pane to configure available Roo installations".
In addition the roo shell's error message status prevents the user from opening other projects in the roo shell. To get out of this the user have to reselect the roo installation in preferences.
In this situation, STS should add the file "com.springsource.sts.roo.core.prefs" and the roo shell should not indicate that there is no valid Spring Roo installation configured, when there actually is one.
One symptom for a project missing the file "com.springsource.sts.roo.core.prefs is that there is no roo-version added to the project name in the "Roo Shell Project Selection" dialog.