Snikket cannot join group when a nickname is already reserved.

[Powsu]

Describe the bug
The bug causes a notification to appear, informing the user that they cannot join a group because a nickname is reserved. This could be abused to flood notifications. The user currently has no way to amend the conflict

To Reproduce
Steps to reproduce the behavior: Create a group chat, create two test account's with the same nickname, have one test account in the group chat, than invite the other account to the same group chat.

Screenshots

Software versions:

• App Version: 1.2.1

• iOS version: 18.0

• iPhone model: iPhone 11 Pro Max

• Server software: Snikket

If your problem involves communication with people using other apps/platforms,
please include details of their app and versions also when possible.

Additional context
I'm migrating my users from a snikket-hosted server to a self-hosted server, when I imported my account data to my new server, I came across this bug. I have attempted to reveal "hidden groups", however that did not show the conflicting group.