there is always this article https://support.safe.com/hc/en-us/articles/25407556603149-FME-and-Esri-ArcGIS-Troubleshooting-Guide but the ssl issues generally relate to FME Flow… not FME Form. 

I suspect you need to enter a redirect URI in the web service definition under your Portal Connection. So go to web connections > manage services and then check that the redirect is 

Perhaps also double check you entered the app id, secret and Redirect Uri correctly from what you created in Porta.  You only need to enter the FME Flow redirect URI when the workspace is deployed in FME Flow

according to https://support.safe.com/hc/en-us/articles/25407724093837-Notes-on-FME-and-Esri-Versions-and-Compatibility  FME 2024.2 is the minimum FME version

What happens if you set verify HTTPS certificate to No?

When you make the connection with the HTTP Caller do you have Verify HTTPS Certificates set to Yes?

Can you make the same http request in a python caller?

I have a vague recollection that the way certificates are handled in the arcgis portal connection matches with the python caller rather than the httpcaller.

PythonCaller gives the same error.

And with Verify HTTPS Certificates set to No it seems to work.

So apparently FME doesn't trust the certificate of our server. How can I fix that?

Have you verified with whoever did the ArcGIS installation that the certificates have been correctly applied? With it being a new installation it might be the problem is there not with the FME installation

If you go to the portal site via a web browser on the same machine that you are using FME on do you get any security warnings?

Yes, 

The portal does have a valid certificate:

did you check the redirect uri

can you check that the version of FME and Arcgis Enterprise are compatible. Maybe the FME Form needs upgrading beyond the minimum FME version of 11.4 enterprise.

maybe someone from Safe software can assist here

The redirect uri in the Web Service is set to: urn:ietf:wg:oauth:2.0:oob

Fme and ArcGIS are compatible, see my original post.

Follow-up, we managed to solve it:

• Downloaded certificates .crt from our ArcGIS Portal through browser.
• Added missing certificates to C:\Program Files\FME\python\certifi\cacert.pem (admin access needed)

Problem applied to FME Flow too, added the certificates there also.