That is very weird... I've seen something similar in wild trees before, but never to that extent. Has to be some pest/disease, or possibly some nutrient deficiency, but I have no idea even where to start. I guess you could rule out the latter if this tree has produced normal crops in the past.

My fast growing Texas 8B Illinois Ev. now has a 2" trunk and 10 ft. height, but only produces fruit that max out at 3/8" thick and 3/4" long. Is that as big as they get? Or, is this size limited by my tree's DNA? Some of it's scions had been grafted on a large m/b tree to see if that would enable them to make bigger fruit, but the fruit size is the same. The birds do enjoy the small snack-sized berries, but my beak can handle bigger berries.

While I am no expert it reminds me of the hackberry (I think) trees that formed a thicket at my home when I was growing up; the leaves had little nodules in them that looked like your berries and they had little parasites in them. I never found out what they were called but they looked similar to your berries.

'Popcorn Disease'.
I see it on IE and other albaXrubra hybrids. Supposedly more prevalent on M.alba and the hybrids - I've never seen it on pure M.rubra - or M.alba, for that matter.
Had some on my IE for the first time, this year.

Wow, I learn something new here every day!

Here is a link that might be useful: Popcorn Disease of Mulberry

Gosh, I can't imagine finding and disposing of all those old berries!
The fruits were normal until last year.
What a bummer.
Thanks for the info.