Since I've installed patch 1.5 I noticed that often npcs and creatures will be litterally 'blasted away' by the finishing blow like an explosion.

Even massive, bulky creatures are interested. It happens with every type of melee weapon and even with backstabs!

Anybody experienced a similar effect? Is there an internal variable or setting I can change to prevent this from happening?

LOLOL so true! Also simple flame spell will send a enemy flying like in the middle of a tornado! Okay, not always but it happens rather a lot.

i couldnt find the mammoth that i sneakattacked using my dagger

So it's something other people have noticed as well.

Out, of curiosity, I tried bringing my saved games to another installation of Skyrim that still uses patch 1.4; this glitch is now present even there, meaning that it has messed with something inside the environment varables. This is good and bad at the same time; bad because, unless I revert to some old file loosing three weeks of game, I'll have all my gaming session glitched regardless of which version of Skyrim I'm using. Good because I hope it's some global setting that can be changed via the console.