I'm trying to disable 5GHz WiFi temporarily, to add a new smart plug in my network. However, I can't save any changes, because the app shows an error message beneath the password: Must not contain space(s).

I don't want to change the password, which has always worked. Also, the app didn't show this error before.

There is no actual limitation on using spaces for passwords, nor a security concern, so why is the app showing this error now?