Mailster uses custom rewrite rules to process certain actions and to route users on the newsletter homepage. If you have problems with links this article can help.
Permalinks are permanent URLs to your individual pages. If you don’t have permalinks setup or your server is not able to handle them your page URLs may look something like this:
If you have Permalinks enabled URLs to look much nicer:
Please read more on the official documentation page.
The link structure of your page is complex so WordPress caches it for faster calculation. This means when the structure changes WordPress has to recalculate the changes.
This can be done by clicking on “Save Changes” on the Permalink page at Settings => Permalink
Once you click the button WordPress will recalculate the permalink structure of your site.