If I even attempt to upload an image to Istock nowadays I get an error message in my brain that says "What on earth are you doing that for?".

The new reduced prices combined with the lowest royalties in the industry killed off any incentive to upload new content to Istock. New stuff hardly sells at all anyway.

I just did a test and it got as far as the photo reaching iStock and needing keyworded. It wasn't a 'real' upload, so I didn't go any further. Were you getting that far?
Getting from the forum page that I happened to be on to the upload page took ages, and from the upload page to browsing my HD took ages, but the actual upload took 'normal' time, all of these being 'impressions' and not accurately timed.

The last person we know of (other than the factories) who took advantage of this was going to 'upload all the photos on his HD' and wanted to exceed the 999 weekly limit. He uploaded an unedited batch of pics, barely described them, keyworded them extremely badly, so they'd never get found for what they are but clog up other searches for which they are irrelevant - except that new files are dropping rapidly in the best match, but they still clog up the 'newest' search (a search which reveals the total lack of keywording standards nowadays), somehow got 803 of his 999 accepted, got 3 sales, at least one of which was used to show how low iS's standards are nowadays, and hasn't uploaded any more since 14th June. I guess it was burnout. 
A salutory tale.

Can't help with DM as I don't use it for uploading, but the direct method seems to be working here, albeit slowly, like most of the site (contributor-side at least).

Now it works. Both: site upload and DM as well.