This IS a bug in your plugin. Of course I don't mean the fact that the rewrite rules have to be flushed after registering a new custom post type, but the fact that you do SO on every single page load. That breaks all other custom content types.
Line 224 in Forms.php
The WP-Forms class constructor calls "flush_rewrite_rules()" everytime the class is initialized. You hook the class to "init", so on every page load it flushes the rewrite rules.
A quick'n'dirty fix would be to remove that line, activate the plugin and visit the "Permalink"-settings once yourself. Now your other custom post types should work again.
A proper fix would be to use register_activation_hook();