Latest report:
I've activated LS again since yesterday afternoon - and it
seems©™ that OS X is running stable again
I've done quite a lot of things with the Mac in the evening and today, but now it runs fast and rock solid with LS enabled.
So what did I do?
After the downgrade to 10.10.2 I've downloaded the
Combo Update from Apple, disabled LS before installing and used this one instead of the App Store updater.
Looks like it helped in my case, maybe it could be an idea for you guys as well to run the combo update.
(But I'll report again, if there's another crash)