InvitationTest#testDeclineInvitation is flickering with the following stack trace:
The screenshot of the failure only shows a login page.
After looking on the test implementation it seems that the test manipulate sessions:
and that in some conditions it might be considered as tampered cookies, according to the logs of the failing tests:
Link to the failing test: https://ci.xwiki.org/job/XWiki/job/xwiki-platform/job/master/1614/testReport/org.xwiki.test.ui.invitation/InvitationTest/Platform_Builds___main___flavor_test_ui___Build_for_Flavor_Test___UI___testDeclineInvitation/