WLAN VPN access from iPhone to Macbook no longer works

[fkuebler]

For the experts, this may even be a silly question, but despite serious attempts, I could not solve the problem:

I have 3 Macs in my WLAN, all with the latest Mavericks 10.9.3, and 2 iPhones, both running the latest Remote Desktop VNC under the latest IOS7. From both phones I could always easily access all 3 Macs via VNC, but since some time I get the following message when trying to connect to one ("MBP", "kuebler") of them (translated from German):

"MBP disconnected. Host: 'kuebler.local' Port: 5900 connection refused. This usually means, that the computer does not run a VNC server on the given port. If you connect via the internet, then the fault may be caused by a not or not correctly configured port. Do you want to connect again?"

With all 3 Macs in System Settings I have turned on "remote administration", and there under " computer settings" I have turned on all 3 checkmarks. Also when comparing I cannot see any differences in the 3 Mac's system configurations. Also all settings in Remoter VNC on the iPhones show no differences for the 3 configured sessions.

Because both iPhones can no longer connect to the one Mac "MBP", I suspect that the problem is within the settings of this Mac. But how can I find out?

Thanks for any hint.