Batch delete of multiple users also removes content

When performing a bulk operation to Cancel a user account and assign to Anonymous, we sometimes get the following error and the user content is deleted from the database, and related images get deleted from the filesystem.

An error has occurred.
Please continue to the error page

An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging Information follows. Path: /batch?render=overlay&id=78&op=do StatusText: Internal Server Error ResponseText:

When proceeding to the error page, the messages are

An error occurred and processing did not complete

Note: The operation works fine when NOT going through Views Bulk Operations, but just one at a time. We have tested this using different methods for https

1. Redirect all non-secure pages to secure pages in apache
2. Use Secure Page module
3. Configure secure pages module to use https for batch and batch/*

Thanks,
Diana