We've finally determined the culprit.

It happens when we run the following command:

PSConfig.exe -cmd upgrade -inplace b2b -force -cmd applicationcontent -install -cmd installfeatures

When the upgrade process fails at about 10% - this annoying verbose problem appears.

We've checked it on 2 "healthy" farms and managed to replicated this problem.

We've finally determined the culprit. (Update: no, I didn't!) check my other answer

It happens when we run the following command:

PSConfig.exe -cmd upgrade -inplace b2b -force -cmd applicationcontent -install -cmd installfeatures

When the upgrade process fails at about 10% - this annoying verbose problem appears.

We've checked it on 2 "healthy" farms and managed to replicated this problem.