BYOND Version:513
Operating System:Windows 10 Home 64-bit
Web Browser:Chrome 86.0.4240.75
Applies to:Standalone
Status: Resolved

This issue has been resolved.
Descriptive Problem Summary:
In 513.1536, unable to log into new accounts using the standalone. Usually selecting 'other' would give the option to register a new account but the input is broken / not available, see

To test,
Nothing has changed with the standalone in recent builds. I'd need source including the byondexe environment you're using to build it in order to figure out what's wrong.
There were multiple issues with the zip you sent, including that the .ico file you use was missing. However once I got the test built, using 513.1526, I discovered that the box looks broken there as well. So the BYOND version apparently has no bearing on this at all.
Lummox JR resolved issue
This issue was determined to be a CSS problem. It may have been caused by the addition of meta and doctype tags that forced IE out of quirks mode, but if so it existed much earlier than was stated.

The default hub.css file will be updated for future builds (513.1537 and later) to avoid any similar problems.