If due to updating a page or setting the page labels directly a label should be removed and this label only consists of digits, this will fail.
Additionally an error message like the following appears in the Jira log file:
There is currently no solution for this. There is a known bug in the Confluence REST API, which prevents the deletion of labels consisting entirely of digits. Feel free to vote for/watch the corresponding issue CONFSERVER-54179 from Atlassian.