Neither .svg nor .ai can be uploaded

Desperately trying to upload my designs, but they fail every single time and I get the red cross. They load with no issues when I convert them to a .png, but I want to screen print them, so they need to be vector files. I’ve read through the guidelines and my files fit all the requirements and I can upload them to other shirt-printing websites with no issues. Where do I (and my files) go wrong?

Try older ai-file versions, like for example cs 5.5. By the way: which other shirt-printing websites require vector files for upload?