Reset Skype settings.
Quit Skype or use Windows Task Manager to kill any Skype.exe process. Go to Windows Start and in the Search/Run box type %appdata%, press Enter or OK button. The Windows File Explorer will popup. Locate there a folder with the name Skype. Rename this folder to something different, e.g. Skype_old. Restart Skype.
N.B. If needed, you will still be able to re-establish your call history. All data are still saved in the Skype_old folder.