Display limited to 800x600 [SOLVED]

[Averell]

Hello,
I used the Pine64 installer to install a "Community Debian Stretch Mate". Indicated version, in July 2018 is : 0.5.15
Beautiful installer, everything fine but the display : it is locked to 800x600, and somewhere a message indicates that the display card is not recognised. I thought I would find a solution and set up the Linux, and used it for some time. But nothing was found to fix the display problem.
Further tests showed that only the 0.6.x versions give me the normal resolution of the display (1280 x 1024 DVI). I tried with :

• Community Debian Stretch Mate (0.5 : Bad)
  
• Community Debian Stretch Minimal (0.6 : OK)
  
• Community Bionic Lxde (0.6 : OK)
  
• Community Xenial Mate (0.5 : Bad)
  

I found later that Ayufan builds really work from 0.6 versions. It is regretful that the 0.5 versions are not yet replaced in the Pine64 installer.

A good desktop experience

Question : is it possible to fix the problem in the installed 0.5 version, or is it necessary to start all over from scratch ?

A strange phenomenon : The defective Debian runs on the eMMC. I tried to flash a SD card with a Bionic 0.6 version. Tested on a second Rock64 : fine. I put the SD card in the first Rock64 : The Bionic is launched, but the display is 800x600. At first, I thought the Rock64 was defective. Then I tried to remove the eMMC card : then the Bionic starts with the 1280,1024 resolution. What can be the reason ? Are some files read first on the eMMC card, when a SD card is present ?

Thanks if someone has a suggestion. I would hate to reinstall everything.
But what I really like with this little thing is the perfect silence.