I remember that you could break tells when too many tell-windows were opened at the same time.
This usually comes down to entries in the UI_charname_servername.ini file.
Either rename/delete the file for the character in question or try to fix it:
- open the file in Notepad++ (for example)
- collapse all (alt+0)
- Select all [Chat x] blocks and delete at least some of them (usually you know how many chatwindows you have on your interface - so I think everything beyond [Chat 20] can be deleted - but you know best)