Debugging an error ("An unknown error occurred")
The error you're showing from the logs is a simple 404 error that occurs whenever someone or something requests a URI on your site that doesn't resolve to anything.  In this case it is /(X(1)S(q325ycazg05lq11ktn1hqjtx))/copyright, which is likely a script probing for vulnerabilities, which is very common for any public website.

We've moved those 404 messages into their own 404.log file in the upcoming Craft 3 so they don't clutter the main log file.

The underlying error message for An unknown error occurred will be elsewhere in your log files.  Sometimes it's helpful to nuke all of your logs (or move them somewhere else), then reproduce the issue as you'll have left to sift through.