If you can reproduce this problem consistently, please attach or link to the file so we can give it a try. You can also send a message to amo-admins AT mozilla DOT org if you prefer.
I tested the file in the standalone validation page and got this error: OpenSearch: XML Parse Error. I looked at the file, and it’s HTML, not the expected OpenSearch format. So, while the server error is in fact a bug, the cause of the problem is that you’re not using the right format for the add-on.
Can you also link to the file you’re trying to upload?
Yeah, for some reason, when I click the “continue” button after uploading and validating the add-on, the page just tries to keep loading and then just shows an empty page. This is a new account, so maybe it’s linked to that?..