X1850 freezing completely, mostly when untouched for a longer period / today during mixing

Hi all,

after about 1-2 hours of bit touching the mixer it freezes completly. Displays turns off and neither the faders nor the potis or buttons react anymore. Sometimes you can wake up the mixer, but mostly I have to switch it off and on again.

Today it froze during mixing.

Any help? Software is V1.5.