@bastianallgeier, I have followed your tip:
just now and got kirby-nightly-2.0.7-20150226163831.zip
With @texnixe
http://getkirby.com/forum/issue-tracker/20141212/fitheight-fitwidth-no-longer-functions-the-same/#20141213135442
this is an error in the docs (a difference to the actual kirby version) or an error within the toolkit.
Hint:
<?php echo $image->dimensions()->fit(100) ?>
echos the dimension of the image, but not the image object itself.