This error should fix itself within 30secs if I remember correctly. If it doesn't, this typically means somewhere in your environment you've got something going into an infinite plugin reload loop - most likely the job server due to missing or different dll versions. Also make sure you're running the latest hot fix as some issues were fixed around this.
Thanks Alex! That makes sense, this was on a development environment so very possible one of us had managed to mess up something somewhere!
I just ran into this today while upgrading our staging environment. Thanks to you and Alex for the explanation. You saved me some trouble! [:)]