Hello,
The wp-admin credentials are incorrect. Please check them.
You use the old version of the theme (latest one is 4.2.1) it can be a reason for the problem, but to give you solid response we need to do some texts. In your case, I would recommend you to create a staging site where we will be able to make changes and not disturb customers.
Also, provide credentials to the staging site (wp-admin and FTP access (FTP hostname, FTP username, FTP password)) in the Private Content area http://prntscr.com/j9gmbb.
Regards