This is very strange but could indicate where to look for similar problems. I dumped the mysensors gateway and node and programed the arduinos with the standard arduino Lora libary to establish a serial device on HA. This worked first time and I could get messages through, no problem. This means that the radios was at least working. Then I reprogramed the gateway and the node with mysensors serial GW and node options. The radios connected first time and I don't think I received a single NACK for two days now.
So I can only think that there must be some radio setup parameters that are stuck in memory or something like that.
Any ideas from someone on this?
Just for others coming here in the future: I changed the connections as suggested by @mfalkvidd , restarted the node and now the radio runs from batteries and the Arduino is powered from USB (phone charger).
Unfortunately this doesn't seem to have improved the radio communication, but this is another story...
If you want more reactions from members, you should provide more information.
what for sensor do you have, because you are speaking about BMP280 and BME280
the lib you are referring to is a specific one for the https://www.sparkfun.com/products/13676
how did you connect your sensor to the UNO?
did you follow this guide already? https://learn.sparkfun.com/tutorials/sparkfun-bme280-breakout-hookup-guide