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(Aha, you're trying to capture on the USB monitor port. See how important it is to be complete in your question? This kind of information is relevant to be able to help you, since we assumed you were capturing on a network interface.)

To be able to capture on USBmon interfaces you'll need to setup a proper File Access Control List (FACL). This is a (relative) new feature in Linux kernels, so that could explain why this was not problem before with 16.04 LTS (disclaimer: I'm not sure when FACLs came into effect in Ubuntu distributions).

This is the command to use to allow yourself to open the USBmon device file:
$ sudo /usr/bin/setfacl -m u:$USER:r /dev/usbmon*

This assume the usbmon kernel module is either build in or already loaded at system start. Otherwise you can load it into the kernel as follows:
$ sudo /sbin/modprobe usbmon

Furthermore, the fact that you only see 5 ports is that these represent the USB busses, not ports. The first represents the 'all busses' monitor device. For further details see here.