Unable to merge a MR: "Cannot find valid SSO session"

Unable to merge a MR.

When attempted to do so, we are presented with the following error:

Already attempted to logout and log in back again, but not success.

This is affecting the whole team. Not sure this problem is restricted to our organization.

Logs

Similar error is shown in Kibana in sidekiq and Gitaly logs:

json.message    MergeService ERROR: group1/sub-group1/sub-group2/merge-request!123 - Something went wrong 
during merge pre-receive hook. Cannot find valid SSO session. Please login via your group's SSO at 
https://gitlab.com/groups/group1/-/saml/sso?token=***

json.error  Cannot find valid SSO session. Please login via your group's SSO at 
https://gitlab.com/groups/group1/-/saml/sso?token=***
json.level  error
json.method POST
json.msg    Internal API error

Possible workaround

Close and reopen MR.