So, this invalid block thing was happening quite often until i discovered something: I was always running my CPU with an undervolt flag for better temperature under heavy loads.
Just wondering, do you ever perform stability/stress testing (e.g. with Prime95 on Windows) when you undervold your CPU?
AFAIK, processors don't have consistent quality. Some are better some are worse, it's what they call "silicon lottery".
You may have picked the one which performs bad with the same underclock/undervolt setting as the others.
The weird part is that only BTC core is affected. This laptop has been a workhorse since 5 years already, poor thing hehe.
Bitcoin Core use your hardware intensely compared with most application, so it's not surprising it only happen with Bitcoin Core. IIRC Bitcoin Core has such warning message when you run it for first time.