Laravel 6, error: Target class [] does not exist. But it doesn't say with class

I have encountered a strange error with Laravel 6. This is the error from the log:

Target class [] does not exist. {"exception":"[object] (Illuminate\\Contracts\\Container\\BindingResolutionException(code: 0): Target class [] does not exist.
at /home/site/wwwroot/app/vendor/laravel/framework/src/Illuminate/Container/Container.php:806)

Following that error, this is in the log:

(ReflectionException(code: -1): Class  does not exist at /home/site/wwwroot/app/vendor/laravel/framework/src/Illuminate/Container/Container.php:804)

Have you ever encountered this problem?

Many thanks