C4C: problems with ERP integration

Hi experts,

we tried to connect C4C with our onPremise ERP System via PI - but have problems:

Landscape:

PI and ERP communicate via RFC

C4C and PI communicate via HTTPS (Service URLs)

ERP and PI are in DMZ because of architectural reasons

The communication of cloud to PI is not guaranteed.

C4C has its won function to test the communication.

This returns the following error message:

Error accessing Service; Service Pink ERROR: Eroor when calling SOAP Runtime functions:

SRT: Processing error in Internet Communication Framework: (ICF Error when receiving the Response: ICM_HTTP_SSL_ERROR") ( )

This error message appears regardless of the authentication method (None / SSLClient Certificate / User&PW).

The Proxy configuration appears correct.

attemped solutions:

Renewed uploading of C4C certificates in PI through basis to exclude false uploading as an error source.

Proxy certificate renewed.

Does anyone has an idea?

Would be very happy about suggentions...

Many thanks & kind regards,

Simone