Can't add license key - NetworkError: 414 Request-URI Too Large

Error:

- Can't add license key.

JavaScript error message:

- NetworkError: 414 Request-URI Too Large - http://example.com/index.php/admin/smartslider3/...&each=function%20(iterator%2C%20context)%20%7B%0A%20%20%20%20var... (and a long javascript function continued in the link)

Cause:

Your Magento is older, than the required version. This exact error is caused by a js file, which is more, than 8 years old. It has problems, and you should update your Magento version.