Hello !
I have exactly the same problem as you: sending the photos goes perfectly well but things go wrong at the second stage; when it comes to processing. Sometimes (but often) only the first image of the sent batch is processed and then the lightbox disappears... as you indicate.
I never found the origin of the problem and I have tried many times with several browsers without modules, etc. I even switched the forum language to English, completely disabled cloudflare, my anti-virus, etc.
I never found any explanation while everything seems to work fine with another administrator of my forum: same operating system, same browser... the only difference seems to be the fact that I use an ADSL connection while he is equipped with optical fiber.
I have of course tried with larger and smaller batches, with different file sizes, etc.
As soon as I send ten pictures (for example) and go to processing, the first picture is well processed (the lightbox disappears) and the other nine files remain in the table "ipb_gallery_images_upload"... 😈
Anyway, personally, I stopped looking for the reason why my other administrator is not facing this problem while we seem to be in quite similar dispositions... we must have missed a parameter.