Hi, I am getting this error when importing a PFX into Portal for ArcGIS (tried with 10.8 and 10.8.1).
ArcGIS Server imports the PFX without any issues.
We are also importing the trusted root certificate, and the intermediate certificate into Portal without any problems.
Debug logging produces no information.
I realize this response may be a little late, but does your SSL certificate have valid Subject Alternative Names? Portal has slightly more restrictions on the format and contents of the SSL certificate than ArcGIS Server does, and may reject a certificate with invalid SANs or other content.