In Exchange 2013, the web-based Exchange Admin Center replaces the Exchange Management Console as the GUI for managing Exchange. As part of this change the process for requesting and importing certificates changes, and you now need to specify a file share and file name when requesting certificates.
When you attempt to create a new certificate and specify a share you have access to on the network, you might see the following error message:
Error: Please use a valid filename when you run the New-ExchangeCertificate cmdlet on server with the -RequestFile parameter. The file should not exist in target folder. Parameter name: RequestFile
To ensure that you can write your certificate request, and read the resulting certificates, you need to assign the correct permissions to the file share you use.
As shown below, grant the Exchange Trusted Subsystem group Modify or greater permissions against your chosen share:
Upon re-attempting your certificate request, the Exchange Admin Center should now be able to sucessfully write the certificate request to the share.