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.
If you have Permalinks enabled URLs look much nicer:
Please read more on the official documentation page.
The link structure of your page is complex so WordPress caches it to for faster calculation. This mean when the structure changes WordPress has to recalculate the changes.
This can be done by clicking on “Save Changes” on the Permlink page at Settings => Permalink
Once your click the button WordPress will recalculate the permalink structure of your site.