Catchable fatal error: Object of class stdClass could not be converted to string includes/theme.inc on line 2742
Clearing your cache goes through all of your code looking for hooks, so something you have installed or updated or written since the last time you cleared cache has this error.

The error technically means that you are trying to use the wrong type of variable in a particular function.  In my limited experience, this usually means you have passed the wrong type of variable to the function before it.

In your case, it looks like you are trying to add a stdClass object $region_content to a string, which is why it is trying to convert it.  You will likely need to dig into $region_content to find the markup you are looking for before adding it to your string.