Unexpected token JSON position ...

:/ why i do nothing and then have to realize, after i printed vouchers for new customers that open cart has an error! I have an error as admin in the admin panel and when i want to add a product in the basket.

SyntaxError: Unexpected token < in JSON at position 872
is in the admin overview panel

and

SyntaxError: Unexpected token < in JSON at position 309
is when i want to put an item in the basked as buyer

Would appreciate a help
I am also considering to change opencart to another or to find a person that can manage (upgrade) and adapt, modify my shop. I am getting more and more frustrated with this shop!!!

Thanks
Sidney