I have had similar problems from time to time with both waves and PA, especially after installing new plugins from either manufacturer. Also when trying to open an existing project I sometimes get the same error message.
What I have found works for me is this. When installing a new plugin install it as normal but do a full shutdown and restart. The plugin manager will always then pick up any new plugins.
With regards to the plugin cannot be opened.. Shut the project down then relaunch Logic and go into the Plugin Manager and turn of the offending plugin. then do a full restart. Then repeat the process and this time highlight the offending plugin and then do your full scan. This for me at least clears this obvious bug down.
Think if you do a search around this forum or Gear Slutz you will find a few threads on why this might be happening. (I cant remember off the top of my head to be honest )
Hope this helps.. :)