Now, I'm not so sure.
I remember having pressed the Escape key at one point,
and that's when I got these errors.
This time I let it run for 2 hours without pressing any key.
So far there were no errors detected.
The thing that confused me was this message:
Pass complete, no errors, press Esc to exit
I thought that would exit the program.
To make sure I ran memtest for over 9 hours and it didn't detect any errors.
So it seems the RAM is okay.
And I'm stumped.