1) Please make sure you are connecting to a registered QSS.
2) Please make sure the database was registered.
3) Please check how many users are currently logging in, is it exceeded number of users for purchased license?
4) Please try to restart server and network router.
5) If all the above were checked, even only 1 user try to login but still prompt the message "You are not allow to connect to this database because all the connections is fully occupied at the moment." , then QNE support person may run the below script:
delete from SYSTEMDATA
where systemdatacode = 'isd390ueg8291mf';
6) After run the above script, the database may need to re-register, after register then user should be able to login.