“Failed security check”-issue

Hello Addify-Team, We run into an “Failed security check”-Issue with the Quick-Buy-Now-Button. It occurs when a new visitor clicks the “Anmelden”-button for “Aufbautraining”: https://www.eft-center-hannover.de/produkt/eft-aufbautraining/ Once you ordered another training, you can order “Aufbautraining” E.g. when you first click the “Anmelden”-button for “Basistraining” https://www.eft-center-hannover.de/produkt/eft-basistraining/ then the “Anmelden”-button for “Aufbautraining” works perfectly https://www.eft-center-hannover.de/produkt/eft-aufbautraining/ This is even true after removing both trainings from the cart and then click the “Anmelden”-button for “Aufbautraining” again: https://www.eft-center-hannover.de/produkt/eft-aufbautraining/ After emptying the cart, closing the browser, opening the browser, the issue is there again. Which probably means that something necessary is missing in the session context. FYI – “Conditional Checkout Fields for WooCommerce” is also used for this product. – This shop runs inside a WP-multisite. The test-site, which is a WP-single-site, doesn’t show this issue. See: https://www.tangodialog.de/produkt/eft-aufbautraining/ – WordPress and Plugins versions are all up-to-date. Thanks for having a look into this! Best wishes, Hendrik The page I need help with: [ log in to see the link] Moved the shop https://www.eft-center-hannover.de from multisite to single site. The “Failed security check”-issue disappeared.