Hello WooCommerce Support Team,
thank you for your support.
I was able to resolve the issue. The problem was related to the plugin “WooCommerce – PDF Vouchers” (version 5.1.2). After reinstalling the plugin, everything started working again — I can now add products to new orders in the backend without any issues.
Just wanted to share this in case it helps others with a similar issue.
Thanks again for your assistance!
Best regards,
Anna from PHORMAT