To clarify the issue: I performed the following steps:
1. created a CA key+cert (selfsigned)
2. created a keypair K
3. signed the public key of K with the CA
4. uploaded the CA-signed cert and the private key of K --> "Your Certificate and Key match"
5. uploaded the CA cert (not the one of K) and the private key of K --> "Certificate an Key do NOT match."
Maybe it just says that for any and all inputs??