If there is any SSO add-on enabled on your Jira instance, you need to add some links from ActivityTimeline into the exclusion list of SSO add-on:
Go to SSO Add-on Configurations
2. Add the following strings into Excluded Paths:
/plugins/servlet/activitytimelineconfig/*,/plugins/servlet/activitytimelineproxy/*,/rest/activitytimeline/*,/plugins/servlet/activitytimeline/*,/plugins/servlet/activitytimeline
3. After it is done, please, go to ActivityTimeline Maintenance page
The Maintenance Page can be found at Jira Administration -> Manage Apps -> left menu ActivityTimeline -> Settings -> Maintenance Page button or via the following direct link:
https://<JIRA_HOME>/plugins/servlet/activitytimelineconfig/configuration
4. Click Restart:
If the issue still occurs or you are experiencing some other problem that is not described here, please don’t hesitate to contact our team at support@reliex.com.