There are different reasons that could cause this error.
The IIS, for example, only allows CSRs created by the user. If you have used an external generator to create your CSR, this may cause the error. To solve the problem, please create a new CSR directly on your IIS and then reissue the certificate free of charge. You can easily initiate this via your customer account. First generate the new CSR on your IIS, then log into your customer account on our website (https://secure.europeanssl.eu/en) and go to the certificate details. By clicking on the "Modify" button, you now have the option of depositing the new CSR and initiating the process of reissuing it.
If you use it in connection with the IMAPd, please check whether the IMAPd has the permissions to access the certificates/keys.
Sometimes it can happen that the file is "destroyed" when transferring from Windows to UNIX, because Windows uses a different character formatting. Please check in binary mode if the lines in CRT / Private Key end with a ^M. You can fix this error with the command dos2unix.