Try with Winbox, instead.
Webfig basically consists in a set of web pages served by the router, depending on the version of routerOS running and how it was upgraded to the running version there have been reports of similar errors/issues with login.
Most of times ROS comes with correct parts (and with all parts) of WebFig. But then browsers tend to cache parts of pages and sometimes they do it in a way which goes against what web server tells. So when elements of web page change, there's possibility for browsers to render it in a wrong way. One can verify if that's the case by opening page with browser set into "incognito mode" (which most of times also ignores items cached in normal mode).
From the looks of it, you have a branding package.
I strongly recommend use netisntall, on windows deselect "Keep Branding" or the appropriate options on other OS.
MikroTik do not provide support for haphazardly done branding packages.
When asking for assistance with something, at the very least, you should specify the version you are using.
You didn't specify your RouterOS version, it should currently 7.2x look like this:
Any package designed to brand RouterOS v6 breaks compatibility with RouterOS v7.
For example, the internal variables like %version% are no longer available.