Contact: 0: Missing format specifier at end of string

The contact form reports the error
0: Missing format specifier at end of string
and the message is not sent.
The form is the same as I have used on my other sites, so I can not figure out the reason.
You can find the form here .
Any idea about how can I fix the problem?

Hi
please go in your config >> system >> debug >> YEs
then tell me the complete message that appears

CEd

The error message is very long: you can read it in the attached image.
Anyway, the Debug option is still on, so you can test it by yourself.

thanks
which version of joomla are you using ?

J 5.0.3 + PB CK 3.3.3

are you using the english language ? it could be a problem in a translation of the original string