Follow the instructions here to find the error message, and post that here.
An quick guess would be to check to see if the symbolic links which are used by the CrossRef plugin are broken. This can easily happen if you extract or git clone the files on a Windows workstation and then copy the folder to Linux server, for example. If the files DOIExportPlugin.inc.php contain filenames instead of link to the actual file content, this will be the issue.