Copy the following command to the browser URL field, replace the values in "< >" and hit Enter: http://<host>:<port>/<contextpath>/rest/scroll-versions/1.0/support/release-publishing-lock/<space_key>

e.g http://localhost:1990/confluence/rest/scroll-versions/1.0/support/release-publishing-lock/PLOCK

To unlock all spaces use this URL instead: http://<host>:<port>/<contextpath>/rest/scroll-versions/latest/support/locks/locked/unlock

Why does this happen?

We lock a space when there is an action from the app that is still in progress, for example publishing a version, deleting versions or when the app upgrades. When any of such actions are interrupted by Confluence shutdown or crash, the lock has to be removed manually.