Thanks for your report.
I've had several reports of this issue as you know. So what I've done today is I've included code in the next version of Wordfence which will go out later today that captures the error being generated and will display it as soon as the plugin is activated. Please upgrade when you see the new version go out today and let me know what the actual error is.
I'm resolving this issue for now and please open a new issue once you upgrade and have the actual error. The reason I'm asking you to do this is because you may receive a different error to someone else, so I'd like to separate the issues.
Thanks,
Mark.