Hello,
Sogo use the openssl library on your machine. So it's the problem with openssl and/or your certificate, not with sogo. Maybe the openssl on debian is too old for your certificate? You could try to read your certificate with the openssl cli to test -> https://stackoverflow.com/questions/8500475/working-with-openssl-to-extract-information-from-a-pkcs12-certificate

I had created the PKCS12 file that I wanted to use for the import with Firefox by creating a certificate export there. It worked everywhere I wanted to import this file, except for SOGo, so I reported this.
But you're right, openssl cannot work with file created by Firefox. I've now created a new one with openssl, which I can read into SOGo.
Thank you very much for the tip about openssl. You can close this ticket.