(In reply to Michel Dänzer from comment #6)
> Please make sure the problem is actually occurring when you capture the
> dmesg debugging output.

I've discovered that the problem occurs when the monitor is switched off.  With
just the monitor off, the "get vblank counter failed: Invalid argument" error
appears.  With the monitor off while playing a video, both the aforementioned
error and "drmmode_wait_vblank failed for scanout update: Invalid argument"
appears.  I'll attach two kernel logs as instructed, one for each condition.