A few things I would check:
1.) Can you ping the HipChat server from your machine (hipchat.com for HipChat Cloud or your FQDN for HC Server)?
2.) If on HipChat Server, verify that you have the correct ports opened per HipChat Server System Requirements: https://confluence.atlassian.com/hc/system-requirements-for-hipchat-server-606306343.html.
3.) Are you using a proxy in front of HipChat? Can you verify 443 https in/outbound to the site from your machine?
4.) If you're still having an issue, please reach out to HipChat support at support.atlassian.com and elaborate as much as possible what you are seeing.
HipChat Server Support Engineer | Atlassian
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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