(с) http://heartbeat.skype.com/

Update: If you're using Skype on Windows or a Mac, these instructions should allow you to sign in again:

Windows Vista and Windows 7

1. Close Skype.
a. Right-click the Skype icon in the system tray (at the bottom right of the screen)
b. Choose Quit.
2. Ensure that "Show hidden files and folders" is switched on.
a. Click Start, type run and press Enter.
b. Type control folders and click OK.
c. Select the View tab and ensure relevant entry is enabled.
3. Locate the shared.xml file.
a. Click Start, type run and press Enter.
b. Type %appdata%\skype and click OK.
c. Delete the shared.xml file.
4. Restart Skype. The shared.xml file will be recreated.

Windows XP

1. Close Skype.
a. Right-click the Skype icon in the system tray (at the bottom right of the screen)
b. Choose Quit.
2. Ensure that "Show hidden files and folders" is switched on.
a. Click Start and then Run...
b. Type control folders and click OK.
c. Select the View tab and ensure relevant entry is enabled.
3. Locate the shared.xml file.
a. Click Start and then Run...
b. Type %appdata%\skype and click OK.
c. Delete the shared.xml file.
4. Restart Skype. The shared.xml file will be recreated.

Mac OS X

1. Quit Skype.
2. Go to the folder ~/Library/Application Support/Skype/
3. Delete the file shared.xml (it will be recreated once you open Skype again, this is fine).
4. Start Skype.

Please note that the ~ sign means your home folder. You can find your home folder by opening Finder and selecting Go > Home from the menu bar or pressing Command (Apple), Shift and H keys at the same time.

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Some of you may have problems signing in to Skype and making calls. We're investigating and hope to have more details to share soon.