It definitly doesn't get recognised by adb when it's in MTP-mode.
So I reset my device to MTP mode & debug mode again to check if it still would be recognised by adb for some reason. I set my device to PTP mode - this fixed the problem.I restarted my device - this did not fix the problem.I tried the following suggestions in this order: UPDATE: Thanks for the repies, the problem is solved (for now).
#MINIMAL ADB FASTBOOT ADB IS NOT RECOGNIZED DRIVERS#
My OS is Windows 7 64-bit, and I'm using a Nexus 4 with Stock Android 4.4.2. My Android device isn't recognized by eclipse nor adb anymore.