Shenzhen_on9

Well. Probably I know what's the problem.  

Since Comnet enforces single login, there is a counter to keep track of the number of concurrent login for each subscriber.
If you didn't logout "properly", the counter is not reset resulting in 'dead-connection', and you cannot login again.  
Normally, there is a job (cronjob) to scan for dead connections and reset the counter regularly.

The support team in Comnet should take care of it.