If I burn with 2.4x instead of 4x, there seems to be no problem. I
wonder what this could mean.
It means that the drive isn't adapting properly to the media, but with
2.4x it falls within the tolerances the reading drive can handle.
I found that a Pioneer A106 never had problems burning Verbatim 4x DVD-R
media at 4x. However 4x media is no longer available, and the same
burner doesn't burn the Verbatim 8x DVD-R media at 4x at all, but it
seems to work (mostly???) at 2x. This is even with the latest firmware,
which improved things a bit but didn't fix them. Pioneer is obviously
not interested in keeping older burner models running, as the last
firmware update was some time back.