The V4S Login fails and a message pops up on the login screen as "System error, unexpected token <in JSON at position 0" is because of the Email ID entered on the V4S Kiosk login screen is not unique.
Please enter the same Email ID which is there in Salesforce to login to the V4S Kiosk application. The V4S Kiosk app looks for the Email ID in Salesforce which is entered on the V4S Kiosk login screen.
The Email ID field on the V4S Kiosk login screen is case sensitive, make sure that the ID is entered as it is in Salesforce to login to the V4S Kiosk app.
Also, make sure that multiple Salesforce contacts do not have the same email ID in the org. If there are multiple Contacts with the same email ID then the login fails, more information here with details on fixation.