Cannot upload images to Backlight thru LR Publisher

I’m trying to update my gallery but I get this message:
Capture d'écran_ 2021-03-31 à 16.23.20
As far as I can tell, these jpegs are normal, processed in LR/Photoshop and exported from the TIFF master file to a jpeg folder within LR. I then drag the file to the Publisher module.
When I try to drag and drop the file directly into Backlight, I get this message:
Capture d'écran_ 2021-04-01 à 12.14.11

UPDATE: I tried, as a test, to upload an image from a previous gallery that already had been successfully uploaded… the same result.

What’s wrong?

YET ANOTHER UPDATE:

I reloaded the Lightroom Publisher Plugin (Latest version: 5.2.1) and the situation is now back to normal.

That is normally an error with the hosting configuration, you will need to increase the max file upload size in the PHP.ini file.