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.
LAN IPv6 Global address is still pingable.