Ever since I updated to 1.6.5 I keep getting page not found when I click on the Plugins page in Wordpress. If I click on other things such as Pages or Posts and then go back to Plugins I will eventually be able to access the page. I also get errors when I try to update a plugin. It says update failed with a list of errors. but it seems to do the update.
If I deactivate WpForo I don't have these problems.
Can you enable Wordpress debug to a log and check in the log for errors?
Also, if you can/want post a url.
Well this is probably a conflict between some plugins and maybe wpforo.
You can also try to disable one by one the OTHER plugins and check.
Also btw you have some js and other errors on your pages. Press F12 to see them.
If you can't access the plugins dashboard, at least press F12 to see the errors or check the debug log. One or the other, will give some information about the problem.