Poppyz makes use of “Pretty” Permalinks to improve readability of the links of courses, lessons, and tiers. If this feature is not enabled there might be unexpected issues resulting from wrong links in the plugin.
To enable Pretty Permalinks go to Instellingen -> Permalinks. Choose Post Name and click Save.
Some servers don’t support this as a default so you’ll have to contact them to allow you to enable it. You can send them this link on how to do it.