Is the sessions freezing and then resuming or being disconnected?

A freezing session is due to a communication issue between the endpoint and Horizon Agent (If you are doing any tunneling it could also be between that device). This can be due to a network communication issue (latency/packet loss) or high utilization on the guest or host that is causing a communication issue (latency/packet loss).

If the session is being disconnected along with that error message it's due to someone logging in with the same username from a different endpoint.

The sessions are getting stuck with black screen it can only be restored by resetting it from the Vcenter console

Following are the PCOIP log snippets if that helps

2019-06-06T08:05:26.865+02:00> LVL:2 RC:  0     SERVER :handle_null_option_msg_from_agent: sending back updated session tag

2019-06-06T08:05:26.865+02:00> LVL:0 RC:  0     SERVER :==> WindowProc: Detected WM_DEVICECHANGE event (0 x 0)

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server mailbox: MBX_SHUTDOWN with agent disconnect reason code (0x5)

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server mailbox: Stopping mailbox message loop

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server main: got terminate application message

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server main: exiting

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server cleanup: tearing down pcoip with agent disconnect reason code(5) and waiting

2019-06-06T08:05:26.912+02:00> LVL:1 RC:  0     SERVER :map_agent_to_tera: DISCONNECT_FOR_RECONNECT -> TERA_DISCONNECT_CAUSE_HOST_BROKER_RECONNECT

2019-06-06T08:05:26.912+02:00> LVL:0 RC:  0    MGMT_SESS :Tearing down the session

2019-06-06T08:05:26.912+02:00> LVL:1 RC:  0      VGMAC :Stat frms: R=000000/000000/000000 T=000000/000000/000000 (A/I/O) Loss=0.00%/0.00% (R/T)

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     COMMON :TERA_PCOIP: SESSION_EVENT=TERA_MGMT_SYS_SESS_EVENT_RESET, disconnect cause (0x105)

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server main: cb_notify_session_status called (mask 0x10) with tera_disconnect_cause (0x105)

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :server main: TERA_MGMT_SYS_SESS_EVENT_RESET with disconnect cause (0x105).

2019-06-06T08:05:26.912+02:00> LVL:1 RC:  0     SERVER :map_tera_to_agent_close_code: TERA_DISCONNECT_CAUSE_HOST_BROKER_RECONNECT -> PCOIP_AGENT_CLOSE_CODE_BROKER_INITIATED

2019-06-06T08:05:26.912+02:00> LVL:2 RC:  0     SERVER :connection_closed: code = 2.

2019-06-06T08:05:26.912+02:00> LVL:1 RC:  0      VGMAC :Session stats: Average TX=0 average RX=0 (Mbps) Loss=0.00%/0.00%

Can you share a screenshot of the pool settings? Is this a floating pool with shared user accounts?

Yes Ben,Its a floating pool of desktops.