What message is throwing InvalidArgumentException(s) in my contact forms? When submitting a contact form it redirects to a WSOD and this is logged:

Uncaught PHP Exception InvalidArgumentException: "$string ("Message") must be a string." at /Applications/MAMP/htdocs/onb/core/lib/Drupal/Core/StringTranslation/


I've tried creating new forms and setting them as default. It didn't work. I've also tried uninstalling and installing the contact module without much luck. Any thought on how to fix it or dig up what exactly is causing it?

Thanks.
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.