Error 500 with Integration

Your software
My Mautic version is:
My PHP version is:
My Database type and version is:

Your problem
My problem is:
Im integrating Mautic with a local partner in Brazil (Hotmart) which sends my leads to mautic after they perform any action in the platform.

Im having trouble with this integration, it works fine after i set it up, but after a few days i start getting this error when it tries to send my contacts to mautic via API

"errorCode": "500",
  "errorMessage": "Internal Error: URI can't be null",

I have to update the integration, and it comes back to life but works for a few days and it shows the same error again.
I tried changing the time it requires to update the API auth but nothing works.

Tried investigating with the Hotmart Platform but they say its on Mautic side.

These errors are showing in the log:

Steps I have tried to fix the problem: