Apparently, there could be an "incompatible key", so I converted it to a proper one using these instructions:
Extract the original private key and public certificate from the incompatible PKCS#12 format file into a traditional encrypted PEM format.
C:\Openssl\bin\openssl.exe pkcs12 -in <PKCS#12 Filename> -out
Where:
<PKCS#12 Filename> is the input filename of the incompatible PKCS#12 file.
is the output filename in encrypted PEM format that will contain both the private key and the public certificate.
For example:
C:\Openssl\bin\openssl.exe pkcs12 -in my_pkcs12.pfx -out my_encrypted_pem.pem
Generate a compatible PKCS#12 file
C:\Openssl\bin\openssl.exe pkcs12 -keypbe PBE-SHA1-3DES -certpbe PBE-SHA1-3DES -export -in -out <PKCS#12 Filename> -name ""
Where:
the encrypted PEM format file generated above.
<PKCS#12 Filename> is the output filename of the pkcs#12 format file.
is the desired name that will sometimes be displayed in user interfaces.
For example:
C:\Openssl\bin\openssl.exe pkcs12 -keypbe PBE-SHA1-3DES -certpbe PBE-SHA1-3DES -export -in my_encrypted_pem.pem -out my_new_pkcs12.pfx -name "my-name"
source: https://kb.globalscape.com/Knowledgebase/11040/Converting-an-Incompatible-PKCS12-Format-File-to-a-Compatible-PKCS12