Just to be sure I have asked them the sequence of actions (though I am not able to reproduce the error even following those)
Before they import from a different course they make sure the courseformat is Grid, so the formats are the same.
FYI: They recently switched to 2.9.1.
In Moodle 2.6 they imported resources etc as well. With the same sequence of actions.
Though the pictures uploaded in Grid never imported with it.
So after they had imported the resources, they always had to upload the images in Grid.
While now they see the grid images coming with the import.
So maybe that's the change? The fact that with importing the grid images import as well, but when they try to change (maybe a day later or something) the server tries to find the images in the wrong place?