Can't login to Magento admin

Question

I have magento installed in a subdirectory. www.domain.com/subdir/magento

This site worked perfectly at one point. I changed nothing, until my client said he could

Answer 1

In Case, that you dont see any cookie named "frontend" or "adminhtml", when you reload the page, the Magento cookie wasn't set. In my case I have a wrong cookie_domain. I used "null" instead of "NULL".

As I set my cookie_domain to NULL in core_config_data, the problem was solved