Can't react to certain posts of other lemmy users, why?

Hi guys,

There is an "issue" that I'm experiencing lately, and I don't know why exactly.

Sometimes a user has a question that I would like to answer for them, so I click reply, but then the instance that user is on asks me to create an account. So it's basically asking me to create another account for that specific instance even though I'm using a Lemmy account to write this post.

My question: Why is this happening, and shouldn't we be able to react to one another?

Im using:

• Lemmings.world via web for my pc/laptop (I use Mint by the way :P)
• Voyager an client which can be found on fdroid for Android.
• Default UI