For all who got stuck, here my codes up to Level 8 (it's the highest I reached, and yes, level 2 is krank... ;)) It seems every level has three codes, sometime I only got two but they all should work the same.
Level 2:BAAWC
WZOEM
ZOAPX
Level 3:BCACU
ZHOVM
Level 4:ZKOTJ
ZCOCI
Level 5:BEAOG...