Voici la réponse faite par le développeur : le problème est lié à une erreur lors de la conception du système. Une amélioration sera proposée dans la future version 3.9.
En attendant, il propose une solution de dépannage pour les versions Moodle 3.5 et 3.8. Ça marche pour certains. D'autres ont des échec.
After some hours of looking into that problem in details I'm still not sure why this happens more often than all of a suddon... I want to explain for the more technical users what happens exactly in the next paragraph. If you are only interested in the solution please read the paragraph afterwards.
So what actually happens is that there is a bar with 27 stripes at the right lower bottom of the page. This is the page number binary coded. Offlinequiz will then draw 3 imaginary lines through this barcode and will find the amount of stripes and the amount of pixels in these stripes. The longer ones it counts as 1, the shorter ones it counts as 0. There is a lot of problems in that and I don't want to go too much into details with it, because this is probably not the right audience. I think it is a bad idea to have so many stripes, because each of them is often only 1-2 pixels wide. This means a small margin of error for scanning/rotation and so on... Sadly I can't just change that, because it would take a bit of effort to support a new page type. I will discuss this further and maybe we will find a solution for the 3.9 release.
For now I can only give some general tips as an advice: First and for almost: In 3.5 we released a black/white conversion directly after the upload
of the file. It was introduced to help with some other import problems. This might be the main cause of this problem. For the people willing to help I would be very grateful for everybody trying to comment
out/delete following line of code:
Line 380 (might be different depending on your release, but it's almost at the end):
You can comment
this out by just adding // in front of it or deleting it completely.
The downside of it is that the finding of crosses might be less error prone. You might change the threshold percentage to 75% or 80% instead in the line above.
For the 3.8 release there will be a chance to change this value in the admin
interface. If this actually helps in your cases as well I will backport it back to the 3.5-7 releases as well.
I would be very happy if someone would try that out if it helps you as well.
Thanks for the wait and all of your support in this matter
You guys are awesome.