URL upload problem

Trying to resolve an error with URL upload. The details: I have two files containing paired end reads parked in a Box folder in both compressed (fastq.gz, about 5-6 Gb in size) and uncompressed (.fastq, about 15.5 Gb). Box can generate a URL to these files which requires no password or login to access. When I copy these into the file upload form (using the Paste/Fetch option), the links are recognized, and the request is processed. BUT, I then get an error message “The uploaded file contains invalid HTML content” and the upload fails.
Any suggestions to fix this?
Many thanks.