I'm sure you'll all be thoroughly bored by the brackets in filenames topic, but in the interest of thoroughness I'll finish this off.

The reason I didn't twig that the filename was causing the issue was because...

a) I don't have the skills to read these errors meaningfully
and
b) I installed the mod (add-on or whatever) with the images already uploaded. Thus when I went to use the Image Manager and attempted to upload an image with an ordinary "no-brackets" name, it failed. I was unaware that it was the existing images (with brackets in the names) that were the problem.

The revelation is kind of hidden when you spend hours trying to upload all manner of file types UNLESS you upload to a spot behind a no-brackets image. And then you are flummoxed because there seems no rhyme nor reason to the intermittent nature of the error.

Anyway, that's all. I'll certainly be naming my files differently in future.