Hey there. There could be several reasons why a specific custom URL isn't available. While clearing cache & cookies resolve most of the issues on browser, try creating a unique custom URL for your channel. Let us know if you see anything else.
Hmm, we'd recommend sending a feedback report directly from YouTube, here's how: https://goo.gl/WdI0BK . Make sure to send from the account experiencing the issue. You may not get a response but this'll help us identify issues.