TBH, I'm no expert with Nginx config (I'm much more experienced with Apache - although still wouldn't consider myself an Apache expert either...) but I suspect that it's either something that Nginix is forwarding that it shouldn't, or something that it should, that it isn't.
I was almost going to suggest that you may need to change the Mayan config to allow connection from the LXC host, but that's clearly not an issue if you can connect to it via curl from the LXC host.
Hmm, perhaps it's worth sharing your config?
TBH, I'm no expert with Nginx config (I'm much more experienced with Apache - although still wouldn't consider myself an Apache expert either...) but I suspect that it's either something that Nginix is forwarding that it shouldn't, or something that it should, that it isn't.
I was almost going to suggest that you may need to change the Mayan config to allow connection from the LXC host, but that's clearly not an issue if you can connect to it via curl from the LXC host.