I have somehow disabled both Webfig and Winbox on my RB751. Luckily, SSH access still works. I don't have any firewall rules that would specifically allow port 22 and disallow the Winbox port. The Winbox "neighbors" dialog lists my router by both IP address and Mac address. When I try to log in, though, the window closes without an error message. I can log in to the webfig home page of the router, but when I log in with a valid username and password the "loading" scroll goes for a long time, and then eventually I'm returned to the login page with an "Internal server error" message. When I use an invalid username and password, though, I quickly get an invalid login message, so I know that I'm getting as far as validating my login.

As best I can tell my troubles began when I checked the "Firewall Router" option from the Quickset page. Of course, now that Winbox and Webfig are disabled, I can't undo that change.

Any help welcome.

I think you change some port for winbox and webfig
From ip, services because cant login in mikrotik server.
You can try to input from mac address for mac of interface you can see in neighbors discovery or you can take mac address from behind device.
Regards

Just bought New (RB951Ui-2nD) RouterBoard hAP. Did all the procedures that was included in instruction page, but... I can't log in to start reconfigure it. By default WebFig:
Login: admin
Password:
And I get a message: "ERROR: Internal Server Error"
Even if i write wrong password I get this message: "ERROR: Internal Server Error"
What to do? I already did a resset, but nothing has changed.

Tried to connect from my Samsung S4 Android

What is solution?