Sudden problem with: exe "set listchars=tab:\xbb\xb7,trail:\xb7"

[Yakov Lerner]

I always had this line in my vimrc:
      exe "set listchars=tab:\xbb\xb7,trail:\xb7"
(It is equivalent to set listchars=tab:»·,trail:·)
Suddenly his line stopped working. (I didn't change it)
It assigns now empty value to listchars:
       set listchars?
       listchars=tab:,trail:

Command L=line  :set listchars=tab:»·,trail:·
stopped to work, too.
It prints E474: Invalid argument: listchars=tab:»·,trail:·

What can be causing this ?

Yakov