[Rude Awakening]

Ok let's try this.
To solve your registered users not being able to view threads check the following.
Go to Usergroup Manager - Registered Users. On the bottom a bar should be added specifying whether a registered user can read thread contents or not. Make sure that is set to yes.
Also make sure the rest of the settings are correct on the top as well.
Can View Forum Yes
Can View Thread Content Yes
Etc.

Make sure you go to the forum permissions and check for the same things.

Now for the plugin, You basically do the exact same as the top, but on the bottom of the page for the Usergroup manager,
Can Read Thread Content - No

Make sure all the other settings are like the registered user's settings in regards to viewing thread content and etc up above the page.

Good luck,

Even I'm confused reading what I wrote so Great Luck with that