pgAdmin 4 error 'utf8' codec can't decode byte...

I have a postgres 9.5 database and a new one in 9.6 ( Windows)
With pgAdmin 4 I try to move tables from 9.5 to 9.6 using *export *and 
using *backup*
In all cases I get an error message 'utf8' codec can't decode byte... 
and a termination code -1
  I tried also with a backup done with pgAdmin 3
The restore on pgadmin4  send the same type of message

PgAdmin 4 v1.1
I tried in two different computers: one with Windows 8, the other Windows 10
-- 
UBO