Photos fail to upload to Blurb without random horizontal blue lines

I will have to defer to the experts to suggest answers for transmission problems, but  you will find these (different problem) suggestions from ​@johnrellis (Legend) that might be helpful at- 

“Try these troubleshooting steps:

- Don't use a VPN to connect to the Internet.
- Temporarily disable all the firewall/anti-virus/security services on your computer, including Microsoft Defender. If that solves the problem, then add a specific exclusion for LR to the services' rules.
- Connect to another network, preferably one serviced by a different ISP. E.g. a Wifi connection of a neighbor, a coffee shop, work, or your phone's hotspot. Sometimes the routers in these networks block some of LR's requests, especially in authoritarian countries like China.”

I hope you find an answer to continue with your books.

I don’t have an answer, but-   When LrC sends a book to Blurb, it is generating a PDF which you don't normally see. So export your book as a PDF to a local folder and review those pages to see if the errors are occurring in your system (and I recommend this for any book).

If your PDF versions look OK then your problem is most likely occurring in the file transmission stage (WiFi?), or at Blurb’s end. 

You might investigate sending a PDF to Blurb instead of the LrC upload, if the problem persists.