Note that there are three different "welches" in German:
- relative pronoun
Der Login, welcher nicht funktioniert." ("The login which doesn't work.")
- interrog. pronoun
"Welcher Login funktioniert nicht?" ("Which login doesn't work?")
"Info darüber, welcher login nicht funktioniert." ("Info about which login (of several) does not work.")
- indefinite pronoun
"Ich brauche mehr Info, kannst du mir welche geben?" "I need more Info, can you give me some?"
Your example is the second instance: "Info, which one of the logins doesn't work."
This should not be confused with the extremely formal use as relative pronoun (1.) "Der Login, welcher nicht funktioniert." ("The login which doesn't work.")
This sounds very stilted to (these) German ears... as though you are either trying to sound like Goethe or trying to mock the one you're talking to.
Before thinking about your question, it has never occurred to me that there can be such a difference in registers between uses of one word:
- relative pronoun formal, to the degree of stilted
- interrog. pronoun neutral
- indefinite pronoun tending to coll. (imo :))
Do any other German native speakers feel like that?