User24 this error happens when you try mentioning a user but do not have the "View user list" permission so Flarum cannot provide autocomplete.
We should probably try to disable autocomplete in the future when we know the user doesn't have permission to search users by usernames.
To prevent the text rendering as a mention after submitting, you can put the text in a code block, For example using ticks
`ssh@ip`
Unfortunately it seems in the post editor itself, whenever it sees @ it will attempt to provide mention auto complete, even if it's a code block.
