When the "Payment Response Text" field says "TRANSMISSION ERROR" this is an indicator that this is an issue with Windcave Please check the below link from Windcave website FAQ:
https://www.windcave.com/support-merchant-frequently-asked-questions-merchant-faq and look for the below question.
What happens when I get a "Transmission Error" response for a transaction?
Your 1st step should be to check if your telecommunications lines are still active and your Keyset hasn't changed. You should then proceed to contact Windcave to help troubleshoot.