What message is throwing InvalidArgumentException(s) in my contact forms?
You're most likely passing an object or an array to a translation function call. The t() function expects a string as it's first parameter (see the function spec on drupal.org). Search for places where you call the t() function. 

More information on how the translation functions are used in D8 can be found on the translation api page on drupal.org. This will give you an idea of how and where you could have translations so you can check those places in your code.