https://support.atlassian.com/confluence/kb/upgrade-to-confluence-4x-fails-with-cannot-drop-the-index-attachmentdataattch_idver_idx/
Upgrade to Confluence 4.x Fails with "Cannot drop the index 'ATTACHMENTDATA.attch_idver_idx'" |...
Fix Confluence 4.x upgrade failure for ATTACHMENTDATA.attch_idver_idx and learn the 3.5.17 intermediate upgrade path.