I have site hosted w/ Ghost that has been (and still is) working wonderfully for several years with a custom theme. I just made some updates to the custom theme to prep for 6.0 and when I try to upload the new zip I receive the following error (for the last two days):
Ghost is currently undergoing maintenance, please wait a moment then retry.
Does anyone have any advice?
If you’re site is hosted with the Ghost team’s managed hosting service Ghost(Pro), your best bet is to get in touch with them at support@ghost.org.
The community forum cannot help much with these kind of issues 
This has happened to me a couple of times in the last several months. I cannot isolate the cause, and did discuss with Support. It seems to go into a loop while uploading. I found that if I tried to upload several times in quick succession I lost access to Ghost Admin and my website froze. Ghost Support had to take action to restore (I was told they purged a cache at their end). Support did wonder if it had anything to do with program I used to zip the files, and I did change. But this had worked for a long time with no issue. I did break up quite a long file in the theme to try to simplify. I was grateful for Support’s help … I am on Ghost(Pro).
I am wondering if you isolated the cause? Or if anyone else is having the same experience?
It’s probably worth checking – if you’ve got the theme set up for development – that you aren’t accidentally zipping up the node_modules folder. Or a .git folder (they’re sneaky - not visible depending on your settings!). I’m not sure that’ll cause this message, but I’ve definitely had errors uploading when I accidentally had a theme file that was 50x the expected size. 