@dc42
Placed R51 back on PCB -> measured resistance -> proper solder joint -> still bad VSSA
Extra solder paste to R50 & R66 -> still bad VSSA
Thought a moment, resistors seem to be ok, ADC according to input voltage reading too, connection to chip still unchecked:
Flux on Atmel chip contacts next to R66 -> No VSSA error anymore but weird temperature readings which are changing when resistor connected
Add solder paste to Atmel chip contacts -> temperature readings are correct again
Thank you for your help.
I am enjoying a working motherboard again 