Thanks for creating this great plugin. It helps me a lot in managing our email campaigns.
I've just noticed a weird bug with permalink, that breaks our permalink structure (returns 404 for archive & single custom post type pages). After digging into the code of the plugin, I see that you use
flush_rewrite_rules() right after
register_post_type() function. That causes a lot of unneeded calls to refresh permalinks on every page load, which might affect other permalink setup.
Regarding the Codex, this function (
flush_rewrite_rules()) should be called only once when the plugin is activated.
I suggest removing that line (
class-chimppress-setup.php file, line 72) to make it work properly (after removing this line, our permalink structure works pretty well).