Cant change theme info in Upfront builder

Hi,

On my multisite i have installed the upfront builder.
Its not network activated, only activated for the subsite where im using it.

I have started creating my theme, but when i want to change the theme info now, then its told me: Oops, something went wrong with processing your request.
Changes is not saved.

I can go to builder and work with the theme without any problems, but im not allowed to change the theme info.

Any idea why this happens?

Best regards,