Symptoms

The following appears in the atlassian-confluence.log:

2014-07-04 14:12:16,842 ERROR [http-apr-8080-exec-10] [confluence.ui.rest.LoggingExceptionMapper] toResponse Exception while processing REST request: 
Hibernate operation: could not insert: [com.atlassian.confluence.labels.Labelling#37289987]; SQL []; Column 'CONTENTID' cannot be null;
 nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'CONTENTID' cannot be null

Cause

 In this case, labeling does not mean the label table. It means the content_label table.

Resolution

• Please ensure that you backup your database before making any modifications
  

• Run the following query against the database:

  alter table content_label modify contentid bigint(20) null ;

  If you are unsure of the database connection parameters, refer to How to find Confluence's database connection parameters.