[Serusers] Error: sorry --cannot open write fifo

[CURRO_DOMINGUEZ]

Hello

I continue with my test of SER. SER is running almost OK, but I have 
some problems. Sorry if this is a newbie question, but is really 
important to me configure SER correctly.

When I try to enter to serweb user_interface, I have this error:

sorry --cannot open write fifo

I can add, delete contacts with serweb, but I can't send Instant 
Messages(nor whit Windows Messenger). I have read  fifo is the 
communication way between ser and serweb, and responsible for instant 
messages. 
 
In config.php, the configuration is 
$this->fifo_server="/tmp/ser_fifo"
And in ser.cfg the configuration is
fifo="/tmp/ser_fifo"

I think is OK, isn't it? 

What can be wrong? 

One more time, thank you very much for your time an knowledge.

Curro