Not sure what the desired answer is, but:
- If you are asking to see what is the actual error is - either change
your environment to testing/development (as suggested so far) or you
can check your logs - writable/logs/log-.php
- If you wish to solve the error, we will need more information, like have
you setup virtual hosts, have you change .htaccess file in public
folder, etc. Otherwise we will be most likely betting (I bet it's virtual hosts).
In any case, a copy of the displayed error (after changing evnironment) or log file will be useful.