Getting a parsing error message

[Ollie]

For some reason, I'm getting this error message when placing an order in my shopping cart: 

parsererror SyntaxError: Invalid character

The shopping cart was working fine until yesterday. Any idea on how to fix the problem?

[abolabo]

system/logs/error.txt ?

[Ollie]

system/logs/error.txt ?

This is the last error message, it's showing yesterday's date:

2016-12-06 3:53:35 - App Warning: AbanteCart core v.1.2.8 Warning: Accessing store with non-configured or unknown domain ( assistech.com:443/store/ ).
 Check setting of your store domain URL in System Settings . Loading default store configuration for now. in /home/xxx/public_html/store/core/lib/config.php on line 210

[Ollie]

This is what I have in system settings:

store URL: https://assistech.com/store/
secure store url: https://assistech.com/store/