I saw this at phpbb.com and confirmed on a 3.0.6-RC2 . It is a minor thing.

To regenerate
1. Login with a normal user account, send yourself a PM containing the line below

[url=http://www.phpbb.com]phpbb[/url]

2. Change PM settings as
UCP -> Board preferences -> Allow users to send you private messages: No

3. Quote the PM sent in step one, it shows uid

[quote="test"][url=http://www.phpbb.com:3k2ax5gg]phpbb[/url:3k2ax5gg][/quote]