When we use Jira on the internal network we have to include the port adress (8080) in the browser. All links to issues in all gadgets on the dashboard include this port adress as well, so everything works fine.
When opening Jira over the internet, no port adress has to be / can be specified. Now in the gadget "Assigned to Me", links to issues appear without port so they work correctly. In the gadget "Activity Stream" though, the links still include the port adresse, as if the page was called over the internal network, so obviously the do not work when used over the internet.