Hi,
Thank you very much for your help!
The size of the files I was trying to upload was from 5 - 12 MB. I was trying to upload multiple files.
It looks like this error I was encountering with uploading files was a temporary issue. I tried to upload a file now, and it worked. I don't know why I was encountering this error, but at least i works now. I still get a lot of warning messages in the error.txt but it does not matter as long as the shopping chart works.