It sounds as though the only way to resolve your current issue is to reinstall Yahoo Messenger. Here's how:
NOTE: By following these directions, all of your archived messages will be deleted. If you'd like to save your archives, please follow the instructions below to save them:
1. Click the Start menu and type Run in the search bar
2. Type in %UserProfile%\AppData\Local\VirtualStore\Program Files\Yahoo!\Messenger\Profiles\ and press Enter.
3. Right-click the folder titled with your Yahoo! ID and select Copy.
4. Click the Start menu and select Run again.
5. Type in %UserProfile%\Desktop\ and press Enter.
6. Click the Edit or Organize menu and select Paste.
Once you've installed the new version of Yahoo! Messenger, repeat steps 1 and 2 above, then copy your Yahoo! ID folder into the "Profiles" folder.
NOTE: If you copy this folder to the Messenger Profiles folder on another computer, your complete Message Archive will be copied, overwriting the Message Archive on that computer.
Now that we've gotten that out of the way, we can move on to uninstalling Messenger. Here's how:
First, uninstall Yahoo! Messenger completely:
1. Click the Start menu and type Run in the search bar.
2. Type in appwiz.cpl and press Enter. This opens Programs and Features.
3. Select Yahoo! Messenger and click Uninstall.
4. Click the Start menu and select Run again.
5. Type in %ProgramFiles%\Yahoo!\ and press Enter.
6. Delete the "Messenger" folder.
Next, download and reinstall Yahoo! Messenger from the following address:
http://messenger.yahoo.com
Once you've reinstalled Yahoo! Messenger, restart your computer, then try to reproduce the problem you were experiencing.