'sudo port clean' fails

[bmayer@…]

I was performing a 'upgrade outdated' and ran into the issue:

$ time sudo port upgrade outdated
Password:
--->  Computing dependencies for qwt61
Error: Unable to exec port: Can't install qwt61 because conflicting ports are active: qwt52
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets

The next step is to do a 'sudo port clean' as near as I can tell, but that appears to have confused itself.

$ sudo port clean
Can't map the URL 'file://.' to a port description file ("Could not find Portfile in /Users/xok").
Please verify that the directory and portfile syntax are correct.
To use the current port, you must be in a port's directory.

[ryandesign (Ryan Carsten Schmidt)]

You did not tell MacPorts what port you wanted to clean.

However, cleaning won't help in this case. The messages is quite clear: you can't install qwt61 because the conflicting port qwt52 is already active. Decide which of those two ports you want, and uninstall the other.