Its very strange unless you are editing the log before posting, errors in my log have more detail about the error.
What I would do is look for this in Web.config:
<customErrors mode="RemoteOnly" defaultRedirect="Error.htm">
<error statusCode="404" redirect="~/PageNotFound.aspx" />
</customErrors>
and temporarily change from "RemoteOnly" to "Off"
Then try your File Manager and if an error happens it should show you the error instead of redirecting you to the generic error page.
See if you get a meaningful error message that way.