How could I change permanent the default UA strings for Netscape and Opera?
Netscape 4.8 into "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8) Gecko/20051111 Firefox/1.5" and Opera 7.54 into 8.54
Modifying "C:\Programme\K-Meleon\defaults\pref\kmeleon.js" does not make the trick.

Go to about:config and search for agent, then change as appropriate. If that doesn't work, then you've got some form of bug.

Modifying ..\defaults\pref\kmeleon.js is the correct way to change preferences for all profiles, newly created ones in particular. But keep in mind, that user-defined pref values stored in your profile's prefs.js (or user.js) override the defaults. To remove a user-defined pref value in order to restore the default value go to about:config, right-click the desired pref and choose Reset. The pref's Status should then switch from 'user set' to 'default' and its Value should switch to the one you've set in kmeleon.js.

Thanks both of you!
Works fine now