Request assistance with fatal error in appointments +

Getting a fatal error on the website which is increasing in frequency. Usually happens when in the backend of the website (admin section). One of the times by client got it = here is the description - went onto my gmail account. I had a appointment needing confirmation. I clicked the link from the email message. I entered my information and went into the site. I confirmed the appointment. I went to click on the pages tab and got this message. Here is the error code --
Catchable fatal error: object of class stdClass could not be converted to string in /home/ecad/public_html/wp-content/plugins/appointments/appointments.php on line 518