Looking for ideas or explanations ...

For a number of years I have been helping a friend with DVD copying/burning, and all have worked well in his player until recently.
These are all legal, privately owned video files, so no copyright matters involved.

Using good brand DVD's I have burned several disks with various formats (.vob, .ts, .mov, mpg etc etc), and all have previously played on friend's player with no problems.
Recently I burned another set of files for him, but all have refused to be recognised by the player ... just the opening blue screen and a 'loading' message whilst the disk 'hunts' (can hear the whirring sound) until it quits.

I've always used DVD+R for him, even though the specs for the player say both + and - are compatible.  Burning has always been at the slowest speed possible (in the past that was best option to get home-burning playing in any DVD player).
All the disks I created play fine in my very old Dick Smith DVD player!

In case the write or read lasers in my PC's disk drive had faulted or moved slightly, I used another portable disk drive for burning. Also tested friends DVD player by playing one of the disks I created some months ago, and it plays fine!.

The only other variance I could think of was the burning software (original was Nero, although Windows 10 burner worked previously too), so used three different burning software apps, all to no avail.
I also tried converting the video files to another format in case the DVD player had some sort of conversion or software glitch within it, but as I said it plays commercial disks and ones I'd burned some time ago.

Maybe Gremlins or covid is living in the player?