Error message, impossible to process this gallery request!

Hello, I have a problem that really annoys me! I installed an old terrain from a former creator, with mods, I modified the terrain for hours and I wanted to save it to delete the mods that I didn't want or were obsolete and know which ones are which because I can't remember which ones I don't like and which ones I want to keep. When I want to save the terrain, I end up with a server error message, impossible to process this gallery request! It annoys me, I spent hours on it, I don't want to start all over again!! I wanted to package the terrain afterwards with Tray importer to only have the mods that I left on the terrain that I want and not the others. Thank you for helping me please