Try uploading this Error.cshtml file contained in this zip to your website at [root]/Views/Shared and then open the File Manager again. You will likely get an error, at which point you should check the system log (Administration > System Log or in the File System at [root]/Data/currentlog.config).
The only time I've seen this happen with the File Manager was here and the problem was related to running in Medium Trust, which we no longer support because Microsoft no longer supports it.
We do need to include this Error.cshtml in the releases. I've created it and I'm adding it to the repo so it will be in the next release.