The problem is that `\href` searches for the `:` but only supports it as a category 12 token, hence you get the error, and can use `\c_colon_str` to work around it.
The fact that this is already an existing question on TeX.SX (though I didn't search for it, so I can't provide a link) tells you that there were at least two people having that issue, which warrants a question to build a knowledge base :P