I see, although I get a bit uncomfortable with the coupling between the nova instance name and the hostname to be set. Perhaps something interesting in the future would be to show the description of the instance, instead of the name, when listing instances in Horizon, thus having the ability to use special characters to give more "user-friendly" names. In terms of this specific bug report, a new one should be submitted to Horizon, and possibly the client (haven't tested), to validate the names before attempting to create the instance, since it deterministically fails after that.
I see, although I get a bit uncomfortable with the coupling between the nova instance name and the hostname to be set. Perhaps something interesting in the future would be to show the description of the instance, instead of the name, when listing instances in Horizon, thus having the ability to use special characters to give more "user-friendly" names. In terms of this specific bug report, a new one should be submitted to Horizon, and possibly the client (haven't tested), to validate the names before attempting to create the instance, since it deterministically fails after that.