Occasionally, the "login abuse code" (designed to detect automated login attempts) may generate false positives. Resulting in the error message: "Login Abuse Check Failed."

This issue appears to occur more frequently on mobile Apple devices.

To resolve this, simply "force close" Safari and reopen it, this will cause the abuse protection code to reload on your device.

​To do this, access the app switcher and swipe the Safari app up and off your screen.
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(Here's an Apple support article detailing the process: https://support.apple.com/en-us/HT201330)

Additionally, if the issue persists, verify that your device has the correct time and timezone settings, as this can also cause such problems.