I would like to ask the same thing. I have a light weight backpack. It is taller than 45cm, but I could remove the frame inside and maybe just put the wire under my jacket if necessary. The rucksack is then without stability and easily compressible.
Do they check in Germany ? I don't want to overfill the bag and will put it under the seat. Have not flown with U2 since 2016 and do this only because this is the best option on that day. :/