Import of client.key file not possible

[joerg]

Hello,
I have created new client certificates. I can import the files client.crt as well as ca.crt. But I cannot import client.key file.
When I try to import it the following error message appears: "empty file or unknown type". I checked the file and it is not empty.
Please tell me what is wrong.
Any help is much appreciated.
Thanks a lot in advance.