Hello,
I disabled all the plugins and enabled the default theme.
This solved part of the problem: the option to enable and disable “use subdirectory for default language” was functioning.
With the default language of my site (es_EC) and this option disabled, I could use domainname/admin. But when I enabled the option, I could not use domainname/admin, always appears domainname/es_EC/admin.
When I changed the order of the languages on the settings page, to English, with the option “use subdirectory for default language” disabled, then again I could not use “admin”. Always appears “domainname/en/admin” on the screen, needs to be “domainname/admin”.
All this without any plugin installed (except TranslatePress). Therefore, I think it′s a bug within TranslatePress.
Best regards