Hello @nikodemsky
Since the 1.3.5 version of the plugin we have added functionality which will deny administrative user logins from third-party login forms. This was applied as an additional protection and such users are being forced to login only from custom login URL. This way we can be sure that website clients are not affected and can log in via front-end form. In case an administrative user logins are exposed by a third party, unwanted administrative access will still be prevented as they will not know the website custom login URL and your website will be protected.
Because of the above security straightening, we had to apply the custom login URL characters limitation. Some custom characters(hyphens in slugs included) are being parsed by the browser and this causes issues with the above-mentioned feature.
Best regards,
Georgi Ganchev