error connecting you to the application.

Hello, 

With Moodle 3.10.6 we have been providing LTI connection to one school for the last 3 months, suddenly during this holiday students has found this notice:
An error occurred when launching the external tool:Sorry, there was an error connecting you to the application.

Others schools are being  LTI connected to us without any problem, so we guess such school has made any change in its Moodle configuration, but the admin says he is not aware of any change.

Any hint?

Thanks in advance