I tested this and it works in FMS15 and FMS14 with Web Direct on Chrome and Safari. Are you sure it is the main login. I did get an ODBC ESS login prompt with one of the files I tested. What happens if you click Cancel?
Going to the link users would use via my desktop computer, it does open a secondary file and prompt for credentials, but on mobile it goes straight to the targeted file. That's okay, as this will only ever be used on mobile. Either way, if I cancel I am left at the WebDirect page with icons of files. Clicking on the targeted file from there does actually invoke the standard FileMaker login box. Bypassing a redirect that is in place and going straight to the targeted file via its IP address also results in the credentials prompt.
Maybe there are some other factors in the hosting of the file that are coming into play, but using the exact same file path to open the file it:
• works as expected in FMP15 on my desktop Mac
• fails in WebDirect on my desktop Mac (prompting for credentials on a related file)
• fails in WebDirect on my iPhone ((prompting for credentials on the targeted file)
In the file's Security, is the chosen account in a permission group which has WebDirect access enabled?
So it works on mobile browsers or FMGo?
This was presented as a WebDirect question.
Also a chance the Server is requiring credentials to view the file list.