Why do all of my self-hosted Actions give the response "Invalid URL"?

[JamesKing]

When I try to test my action in the configuration screen, or when I try to actually use the action, it always says Invalid URL.

This is all I see in my docker log for the Action Hub instance:

info: Starting request. url=/, ip=::ffff:192.168.62.3, statusCode=200, instanceId=742792674bb07674057f327d35cdf32a, webhookId=adb6065a26199172e5f78e49440219d4
info: HTTP POST / statusCode=200, url=/, method=POST, httpVersion=1.1, originalUrl=/, , responseTime=6, url=/, ip=::ffff:192.168.62.3, statusCode=200, instanceId=742792674bb07674057f327d35cdf32a, webhookId=adb6065a26199172e5f78e49440219d4

Thanks