I have to add some information: I can't use the lastest firmware because the modules are spread and so out of my reach :( I flashed all LoPy modules in April so that's the latest firmware I'm currently using.
While searching the web for my problem I found this:
https://github.com/Lora-net/LoRaMac-node/issues/104
It says that there is a bug in the SX1276 driver and so maybe in the SX1272, too. It misses a division by 4of the SNR value... And thats the factor I need. So: Does anyone know if this bug was a thing in the LoPy's too and had been fixed in the last weeks after my last update? I can't find anything about it in the last changelogs...