If there is any SSO add-on enabled on your Jira instance, you have to add a couple of links from ActivityTimeline into the exclusion list of SSO add-on.
Below you can find a short guide on how to do it on example of EasySSO add-on:
Please, go to Jira Administration → Manage Apps → On the left panel click on EasySSO:
2. Add the following strings into Excluded Paths:
/plugins/servlet/activitytimelineconfig/*,/plugins/servlet/activitytimelineproxy/*,/rest/activitytimeline/*,/plugins/servlet/activitytimeline/*,/plugins/servlet/activitytimeline
3. Click ‘Save’.
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.