Whatever you just did seemed to fix the problem. I'm a poor student but @VITAS I think I can afford to send $1.00 a month your way for being so responsive and available to troubleshoot these issues!
Also got this error, don't know if it helps:
Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /mod/172/KSP Interstellar Extended/download/1.9.11.
Reason: Error reading from remote server
It still seems to be having issues... I'm still getting a 503 for every mod I've tried to download, even after using shift+reload; even when it works and *starts* downloading, it stops after a few megabytes with the same 503.