Unable to verify the first certificate

Your software
My Mautic version is: 2.15.1
My PHP version is: 7.2.29

Your problem
My problem is:
I try to link Integromat with Zapier and followed the instructions on https://support.integromat.com/hc/en-us/articles/360012769419-Mautic to create a set of API keys and enter them in the Integromat software. In Integromat I then setup the connection as described in the instructions and it will ask permission to access my Mautic installation but then a few seconds later this message appears: unable to verify the first certificate.

These errors are showing in the log:
There are no errors showing in the log files (both server and in mautic)

Steps I have tried to fix the problem:
Nothing so far, the website does have a valid ssl certificate installed so that could not be the problem.