SC6.2 crashed by too many empty user sessions

now the system crashed very frequently,(about every 20 hours), please help!

 

there are a lot of empty user name&device sessions in system status. and meanwhile many system locks by user RCOPERTOR, JETOPERTATOR, HPSX .... all these users are used by integrated system. they all using webservice for integreation with OVSC.

 

we have inactive timer set to 60 and timeoutlimit in sc.ini set to 3.

 

screen shot for system status and locks is attached.

 

I can't find any valuable information in logs. could anyone help on this?