WooCommerce made a major change to the REST API in version 3.6. This is causing plugins to throw errors when the REST API is used, these errors then appear in the POS.
Please go to WP Admin > WooCommerce > Status > Logs and check for any error messages since you updated to 3.6. In particular look at the fatal-error-log.
It will take some time for all the plugins to adjust to the new WooCommerce update, so it may be worthwhile rolling back to 3.5.8 until these problems are ironed out.