After some more research, my host tried changing the php error reporting to 2039 (not sure what it was set to before). Now I get a new error:
File is empty. Please upload something more substantial.
I’ve tried uploading a variety of different sized images, but I always get that error. I did a search on the error and found that for some it occured if file_upload was off, but I just checked mine again and it’s clearly set to “on”.
Still, this does feel a bit like progress… at least it’s not telling me it can’t write the file anymore.