I was sloppy when installing an add-on the other day on a client Community Server project. I installed this particular add-on a couple of times before and didn't feel like walking through the README.TXT. When I loaded the site I got this.
Getting past the Community Server Critical Error Page has the potential of being time-consuming, but rather than analyze .config files or backtrack your steps consider checking the latest entries in the cs_exceptions table. Doing so in this instance quickly alerted me to a missing configuration file that my new add-on was expecting to see.