Peace MindFlowers68!
That sounds incredibly frustrating, especially since you've already tried the standard troubleshooting like resetting and clearing history. It sounds like the app or browser session is timing out as soon as your phone goes into "sleep" mode to save power, which kills the upload process.
Try this:
Go into your phone settings and temporarily set your "Auto-Lock" or screen timeout to "Never" while you're doing your uploads. This keeps the connection active until the job is done. Also, if you're using the browser, try switching from mobile data to Wi-Fi (or vice versa), as a small dip in connection can cause those errors.
Give the screen timeout trick a shot first it's usually the culprit for large photo batches!
Happy Growing, Growmie!