If you are using a caching plugin on your WordPress installation or a caching module that is installed on your server, such as varnish for example, you may experience problems with the Wishlist Member official API responses.
Caching solutions that operate on your entire website without distinguishing the kind of information it is caching causes the Wishlist Member API not to authenticate itself and will cause problems with many API operations.
In order to solve it you will need exclude all HTTP requests that have /wlmapi/2.0/ in url.
This is not a problem of any of our Wishlist dedicated plugin or Wishlist Member itself as caching plugins are causing these issues with all plugins that generate dynamic information.
In some cases CDN solutions such as CloudFlare also use caching when delivering the information and the effect they have on Wishlist Member API is the same as specified above.
The solution is also the same and you will need to consult with CloudFlare on how to exclude the above URL.