It looks like the API for your module is not responding appropriately. First, it's returning HTML, and second, it's sharing an error and stacktrace because something is wrong with the CWP server. This issue isn't caused by Blesta, but by the server it's attempting to connect to. You should look into that CWP server and resolve the error that's mentioned because it's not running correctly.
Type: ErrorException
Code: 8
Message: Trying to get property of non-object
File: /usr/local/cwpsrv/var/services/api/v1/index.php
Trace
#0 /usr/local/cwpsrv/var/services/api/v1/index.php(0): Slim\Slim::handleErrors(8, 'Trying to get p...', '/usr/local/cwps...', 0, Array)
...