We just encountered a critical problem with the Paymatch plugin on our shop. I had to deactivate the Plugin to restore Shop functionality.
Symptoms are as follows:

when placing an order in the frontend the user sees the checkoutpage again with additional Internal Server Error displayed in a infobox.

when i turn on WordPress debug messages it shows a SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data error in the frontend and in the webconsole (Firefox) I can see a Unable to fix malformed JSON in checkout.min.js:1:8274 message. checkout.min.js is a core woocommerce file.

In the Paymatch plugin logs I see an add orders post failed with WP_Error: cURL error 60: SSL certificate problem: certificate has expired everytime a order is placed

additionally when reactivating the plugin after deactivating it it shows failed to create orders_queue table, failed to create transactions table and failed to create matches table

Our sites Certificate is valid and has not expired yet.
Deactivating the Plugin solved all errors.