Update:
I downgraded to WordPress 5.4 and now it works again.
Best regards,
Mette
Problem solved. I deleted the coupons and created them again.
I found the old version of WooCommerce (version 3.5.7) and now the site is working again.
I think, I have the same problem with https://www.lejdirekte.dk/ – It changed when I updated WooCommerce. Did you update WooCommerce, when it happened?
The error shows that i-craft is not compatible with the new version of WooCommerce 🙁