Because clan strings can appear in game, they are subject to to additional validation by our platform partners. Unfortunately, due to a bug in bungie.net, the wrong error message is being returned so you can't see what is being flagged by the validator. While that bug will be fixed soon, your particular problem should be solved if you remove the "bungie.net" from your mission statement when you are attempting to add your motto.
Well, that worked! Thank you so much!