Please make sure you are using the latest version of B2J-Contact, recently we have added multiple fixes in the extension to work properly with Joomla 3.5.1.
If you have upgraded Joomla version at your site to Joomla 3.5.x then kindly use the latest one.
In case you still got this 500 error after using the upgraded version, then enable error-reporting to maximum in Joomla global configuration and then try to submit the form and let us know what error appears on the screen.
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