Seems like TFTP has been killed in version 5.0 RC6.
When booting my PC machines, (PXE IPv4 boot menu through TFTP), PXE was not able to load TFTP files on RB750G.
Rebooting the router does not help.
I notified as well that Winbox loader (2.2.18) was not able today to show the router LAN IPv6 address. It does show only the LAN Link Local IPv6 address (fe80…). This was working yesterday after installing RC6.
under /ip tftp you can add rules and there you can add ip-address field that specifies from what addresses it is allowed to connect from to this tftp rule. So for now, you have to add address there, it will be resolved in rc7.