I just tried to access the RB3011 first time, with the existing credtials of available user.

Unfortunately I am continously getting error of bad username or password, though it is accessible from other machine, with same credentails, and from same LAN

Can that be MAC address bound/restricted and how to allow that?

Are both of PCs have the same winbox version?

Yes there are ways to limit winbox access by IP address!!

Well in both cases the IP assignment is through DHCP Server of RB3011 router, and not fixed at the terminal which can access it.

Are both of PCs have the same winbox version?

Well we are directly accessing it though web GUI, though i tried it with v3.27 too as well.

Then check IP -> Services, System -> Users and IP -> Firewall -> Filter (input chain) sections to see if access is not restricted to some ip's (whether for this user only, or to device in general).