RBR20 Orbi guest network doesn't allow special characters?

I used to have special characters in wifi passwords, but didn't try to change passwords recently.

If you tried this and you're sure that it does not work for the guest network, then my only explanation (just a guess) would be that: Not all WiFi capable devices (such as various smart accessories and gadgets) support spaces and and few of them not even support special characters in wifi passwords! In your main network you know your devices and have control over them, but in a typical guest network your guests can have any type of device that can happen not to support special characters (mainly spaces which is tricky for many devices).