I've been getting two errors:

warning: htmlspecialchars() expects parameter 1 to be string, array given in .../includes/bootstrap.inc on line 857.
warning: htmlspecialchars() expects parameter 1 to be string, array given in .../includes/bootstrap.inc on line 857.

To debug this I added this code to line 857:

if(is_array($text)){
  drupal_set_message('bootstrap.inc 857 - $text: '.print_r($text,true));
}

which returned this:

bootstrap.inc 857 - $text: Array ( [0] => Open a new case assigned to a particular project. [1] => )
bootstrap.inc 857 - $text: Array ( [0] => Create a basic project for use with Case Tracker. [1] => )

I tracked that text to the casetracker: casetracker_basic.module file; function casetracker_basic_node_info() on lines 24 and 31.

Line 24: 'description' => t('Create a basic project for use with Case Tracker.') 
Line 31: 'description' => t('Open a new case assigned to a particular project.')

The t() function uses the checkplain() function which in turn uses the htmlspecialchars() function.
But this code looks fine; calling a string and not an array.

Further research is required. I'm guessing t(), checkplain() or htmlspecialchars() is being used again when looking for the casetracker_basic_project or case descriptions. The broken 'Add Case' and 'Add Project' buttons certainly seem like the place to start.

Because of that, I'm not sure if this is a CaseTracker bug or OpenAtrium bug yet but I wanted to point it out in case it's happening outside of openatrium as well.
I've posted this message here in the openatrium community forum as well: https://community.openatrium.com/issues/node/2560#comment-7012