Problem with wildcard certificate : Certificate was found but is not valid for use with Exchange Server

Hello,

I'm having a problem to enable a wildcard certificate for Exchange 2013.
I have all 3 files for the certificate (pfx, p7b and cer), and the password for it

When I import the certificate I get a message that the import succeeded.
Import-ExchangeCertificate -FileData ([Byte[]]$(Get-Content -Path \\localhost\Certificates\wildcard.customer.be.p7b -Encoding byte -ReadCount 0)) -PrivateKeyExportable $true -server Exchange01 -FriendlyName "owa.customer.be"

However When I perform a get-exchangecertificate I don't see the certificate in the list.
And when I try to enable services on it, I get the error below :
Enable-ExchangeCertificate -Thumbprint "50328DD4E2344121CFC7A78F3A49A80FE35EAB76A" -Services IMAP
> A special Rpc error occurs on server LABEX01: The certificate with thumbprint 50328DD4E2344121CFC7A78F3A49A80FE35EAB76A was found but is not valid for use with Exchange Server (reason: PrivateKeyMissing)

I've used the same certificate for our exchange 2010 environment before without any problems.