plantuml app diagram always shows broken icon on page

[Ward Van Heddeghem]

I've just installed the "PlantUML Diagrams for Confluence". for our  cloud confluence to test drive it (new setup, no other apps installed so far).

Problem: I can create a diagram when inserting a /plantuml macro, and it renders correctly in the plantuml editing popup.
However, when clicking "Save" in the popup to go back to the page, only a broken icon is shown. Same when  publishing the page. See the screenshot below.

Question: How should I troubleshoot this?

Some background on what I have already done

I've used confluence (and plantuml in confluence) extensively and successfully before, but it's my first eperience with adminstration of confluence (installing marketplace apps myself etc). I am trying to get plantuml working on a new instance I will be administering. I am a site-admin group member.

What I already tried/did:

• The apps documentation (https://stratusaddons.freshdesk.com/support/solutions/articles/48000618047-introduction) didn't help me.
• The app's license is trial but valid (until sept 2020).
• I don't see any app settings or configuration page.
• In chrome's developer tools, I see the below console error when loading the page. I don't see it on pages that don't have the plantuml macro. Related?
  "Error while parsing the 'sandbox' attribute: 'allow-storage-access-by-user-activation' is an invalid sandbox flag." 
• I checked my atlassian group membership (under https://admin.atlassian.com/), my user account is part of the following groups: confluence-users, jira-software-users, site-admins

[Ward Van Heddeghem · Accepted solution]

It seems to be a 3rth party cookie blocking issue. I worked around it by allow 3rd party cookies for the atlassian.net site

[Boris Jockov]

The underlying issue seems to be using cookies to authenticate request for loading the image.
So I can confirm the supplied workaround is the correct one.