have you checked the solidworks kb at customer portal.solidworks.com or checked the forum? there are some good articles about this.
the message means some flow is exiting and some is entering through that boundary condition. it may occur during the intial iterations and then go away. that is good. if it remains during the calc, you should probably move your boundary condition away from that location (extend the pipe for example) or potentially you need an external analysis depending on the case.
note, this is likely numerical vs physical. as noted above, you will need to try different boundary conditions if the result remains. also note, environmental pressure is better at handling this condition.