Problem

After disabling a user, you're still able to mention it. Sometimes when trying to save a page containing a mention to a disabled user you receive:

• Can’t reach the server. Check you’re connected to the internet and Confluence is up and running

Diagnosis

Cause

The username suggestion still cached in the browser. Notice that odd things happen depending on how many characters you type after @, that's because not all combinations are cached. 

Resolution

Clear your browser's cache completely.