I receive empty page in my admin panel stating "Invalid Serial Number"?

From CashCrusaderWiki

Like the message states, you have invalid or missing serial number.

To fix this, get your serial number from your welcome email and run following query in your phpMyAdmin:

REPLACE INTO system_values (name, value) VALUES ('serialnumber', 'YOUR_SERIAL_NUMBER_HERE');

Note: You need to replace your serial number to the query.

Note2: If you have MySQL prefix, you will need to add it before system_values table name.