I can't add new product on the web license manager

[moo]

I am unable to add a new product on the authorization management page.
After filling in the product information and clicking the [Save] button, the page displays a Loading message at the top, but it seems to be stuck and unresponsive.
I opened the browser's debugging mode and found that it returned a "500 Internal Server Error." How should I proceed?
PS: It doesn't seem to be a database issue because I can successfully add a new agent account.

[Admin]

Did you check PHP error log?