I'm encountering the same problem.
It's only affecting one of my bases currently. My other bases are able to have files uploaded correctly.
The base that is affected is my most populated base. Not sure if record count or total storage amount is a factor, but I'm nowhere near the limits of what my plan allows.
I opened a new attachment field and was able to upload documents with no problem. It's a shame. The previous field (the one with the fault) was linked to a series of views and filters. Hopefully Airtable knows what this error is about.