With reference to https://community.atlassian.com/t5/Jira-Core-questions/Confluence-page-links-you-create-show-Failed-to-load-on-the/qaq-p/345696
I'm afraid this issue is back again for Jira 7.4.1 and I'm running with Confluence 6.3.1. When my application links are setup with Oauth,(Impersonate) and I select 'Linked Pages' on the Sprint backlog board, it shows link: "Page failed to load" but if I select the link it takes me to the correct Confluence page and with no authentication prompts. If I switch the Application Links to just Oauth and I select the 'Linked Pages' , a dialog box pops up asking me to authenticate and then asking to Allow/Deny the linkage. Once I do that, the "Linked Pages" properly shows the title of the linked Confluence page.
As a workaround, it recommends to disable JIT optimization. I will try it but this reflects poorly on Atlassian's regression testing that this issue keeps reoccurring.