@bloggerboy it is not _maintenance.html the file which you need to delete. Auto-upgrade also creates a umaintenance.html file, please check this page: http://b2evolution.net/man/maintenance-html

Please update the 503 page error message. It says to view the source code for additional details, and the instructions are to delete or reneame _maintenance.html

Otherwise, it's all moot now.

Thanks

@bloggerboy a clarification note is already included in the code. Thanks.