I'm using the RB750UP as a bandwidth shaper for a small office building, so I removed the default setup including all firewall rules and created some simple queues and static routes.

I can use Winbox on the switch interface but not the WAN interface. On the switch interface, I use the default address of 192.168.88.1. On the WAN interface, I use the address 172.27.236.150. When connecting to the WAN interface immediately after a reboot, Winbox connects, shows me the last items I was viewing including data. If I select any item from the menus, the data is blank and I'm disconnected after a few seconds. After that, I'm not able to reconnect to the WAN interface.

I'm running routerOS 5.17, but it did the same thing with routerOS 5.6.

Any ideas what would cause this behavior?

How did you remove the default config, using the script?
I would recommend doing it with this command: Are you trying to connect only with winbox? Via mac-address or IP?