The question seems to contain a hint for a workaround that worked for me, so many thanks!

The issue seems to be specific to sdk 6.0.135. I've had exactly same setup as the topic starter on my local machine and run into exact same issue. Furthermore it started to show up on any(!) project, even blank VS2019 console hello world template (targeting net5.0).

• I removed the 6.0.135 sdk installation and console app would work, yet my original app targeting 6.0 was obviously unhappy
• I reinstalled 6.0.135 and the problem re-appeared
• I removed 6.0.135 and installed 6.0.133 => it works and suits me

Also my colleague having 6.0.131 on his machine saw no trouble. Still really curious about mechanics of that, especially given I've had this setup untouched on my machine for many months and then it just showed up roughly at the same time as others