Last tested: Apr 10, 2020
This error generally means that Looker tried to have the Chromium process load a page in order to capture its contents in a PDF or PNG, but the page did not fire the DOMContentLoaded event within 5 minutes. This most commonly happens when loading the actual content (dashboard, look, etc)
Other than unexpected issues, the main culprit of this in the past has been certain outbound requests being black holed, so they hang indefinitely while the page is trying to load until this timeout is hit. Looker will use localhost for requests*, but it’ll also make requests to some external servers (e.g. ping.looker.com, google-analytics.com, etc).
If you are an on premise user, check if you are allowing all outbound traffic. If not, you can load the dashboard, look or explore you're trying to render in the UI and look at the network tab in the Chrome dev tools. You should try to curl every URL that isn't your host URL (so in the example below, everything that doesn't start with master.dev.looker.com)
If there are any that you can't reach, you'll need to whitelist outbound traffic to those domains.
* Only exception to this is if the user is using the
--ssl-provided-externally-by=<s> startup flag. This is a somewhat rare setup, but worth checking by running
ps aux | grep java and seeing what the startup flags are if all else fails. In the vast majority of situations if there is something wrong with Lookers' ability to reach that URL, though, it'll fail earlier in the process with a different error.