Redmine/Internal Server Error
This error can happen in various different places for different reasons.
when attaching a file
If this error appears when you try to attach a file, it may be that the upload folders are owned by the wrong user (my theory is that this happens when Redmine is installed from a package and run as a virtual server -- the processes set up by the package assume it is running as the default Apache user,
www-data, regardless of whether this is the case, and periodically "correct" ownership of those folders).
To fix: check ownership of the user data folders.
when loading a page
If you get this error instead of a web page, you'll need to check the server logs for a more specific message. It could be incompatible character encodings: UTF-8 and ASCII-8BIT, for example.