Steps to reproduce:

• create user with username 'test user'
• login as 'test user' and create a personal space
• Browse > Pages, Recently Updated - <confluence>/pages/recentlyupdated.action?key=~test+user

The error is:

recently-updated: '~test' is not an existing space's key

Update: This no longer produces an error but displays "No recent updates found." Like Jeff said, this is because Confluence splits on spaces resulting in a query with 2 spaces. The suggested fix below works however, has an edge case (query with a personal space and global space mistaken for a personal space).
To address this problem, Confluence needs to be able to escape space delimiters.