I needed to update my Octopi instance because pip and setuptools etc. were out of date, so I used the backup function as part of Octoprint to create a zip file and download it.
Fresh Octoprint install, and try to upload the backup file to Octoprint and it doesn't go. I've had a look, and while it says it may take a while (zip file is 3.7GB) however there is no network activity and it just sits there.
I'm about to try a new install of Octopi, and I did image the old one before overwriting it, but if anyone has any suggestions of other methods or fixes they would be appreciated.
Edit: Just realised, this is not actually a plugin and is part of Octoprint. Changed the topic accordingly.
First do you have enough free space for the backup?
Did you try an other browser?
I have enough free space, and have tried edge as well. I am wondering if it is to do wth the file size as the edge window appears to crash (It reloads back to the homepage). Are there any important parts if I try and cut some of it out like the timelapses/previous files uploaded?
I've got another idea which you could test before.
Upload the file yourself to your pi (or mount the card on your linux pc).
You may use WinSCP on windows for the upload.
Then restore it in ssh http://docs.octoprint.org/en/maintenance/bundledplugins/backup.html
I've cut the timelapses out of the zip and it's down to 400MB, seems to be working now. Some investigation for max upload size?
Great. 
I have no idea regarding the file size.
You can also restore the timelapses with WinSCP afterwards if you want.