Hi Doug,
I almost predictably encounter this problem with my "This day in history" uploads, and the next attempt I will screenshot my work as I go, and it if fails I will forward the requested information to you for troubleshooting.

I usually have no troubles with this or any other forums and am respecting the upload file size limits when doing so, but I will try my best to help out with finding the root cause which may lead to a fix.

Wild speculation:

My upload is being interpreted as some sort of "cyber attack" to the site and my IP is being blocked for a timeout period to protect the site.

Crude work arounds include:

1. Using another device on the same network for accomplish the task, (mobile smartphone via tapatalk app)
2. Waiting 1-2 hours until the site responds and allows me to view the site and resume viewing and uploading. (2nd try of the exact same task is always successful)
3. Re-setting my modem, and reconnecting to my ISP. (Allows instant restoration of milsurps site access.)

However I know vague information is useless for troubleshooting. I will try to replicate the error this evening.