My MODX error log is full of this:
(ERROR @ /core/model/modx/modcontext.class.php : 128) PHP warning: array_merge(): Argument #1 is not an array
It's caused by $resource->Context->getOption() in the /lib/StatCache.php file.
I fixed it by adding $context = $this->modx->getContext( $resource->get('context_key') ); and replacing $resource->Context->getOption() with $context->getOption().