I want to upload a map I used for long to MC Realms. While verifying the Data, I get the Error (500) message.

Other maps work fine, just this one doesn't. It was originally created using Minecraft 1.8.
I've tried to first create a world and then overwrite it, to no avail. I've created new worlds, exchanged the chunk data; same result. I've made a copy, set it to peaceful; same, same.

The map does not have any texture packs affiliated, nor any mods. it works fine in Singleplayer, no corrupted chunks or anything.