Underfloor heating - return warmer than supply?! What to do?

Hello everyone,
A few quick facts about our single-family house:
- 157 sqm, detached
- Guntamatic pellet heating
- basement
- KFW 55
- heating curve 0.5
- moved in Nov 2018

Which parameters should I adjust first if the return flow is warmer than the supply flow?

Hi
That actually can't be the case when the boiler is burning (generating heat).
Do the measuring points show the correct temperature?
Is solar-heated water possibly flowing from the buffer to the boiler?


Olli