The SlugEntityNameValidation aka "Preformatted names" page naming strategy thinks that a page name consisting of only a single letter is not valid.
To reproduce, activate that page naming strategy in the wiki admin, and set both "Transform names automatically" and "Validate names before saving" to "Enabled".
Then try to create a page with name "A". An error message appears that this is not a valid page name.
Easy workaround: disable "Validate names before saving".