It is very probable that there may be some clients which don't close connections. In that case adding some kind of "FE" timeout to close them may help.
1. Go to "Global Parameters -> Global Properties" and activate the "Define Default FE Timeout" checkbox. For the amount of time you can choose 120 seconds.
2) Restart the instance.
In addition to this, the last two versions of QCVDS (6.0.4 and 6.0.5) have fixes related with 'Simultaneous connections limit' issues. And, regarding the Administration port, these versions already include a "Use FE Timeout" checkbox at the 'Administration' section, in case you are experiencing a lot of connections to this port that are not closed.