We spent a while wondering this as well, and at least for us the problem was caused by the default header that the sale order webkit report is using.

The "Base Reports Portrait Header" header (provided by base_headers_webkit module) makes a reference to ${company.partner_id.address}, which seems to cause the issue.

As a workaround you can pick base_headers_webkit's header "Base Minimum Portrait Header" which does not include the address field. Not a perfect workaround, but at least you can get past the error and start to work on the actual reports.

You can replace the header by opening sale_order_webkit/sale_report.xml and changing the webkit_header parameter from webkit_header="base_headers_webkit.base_reports_portrait_header" to webkit_header="base_headers_webkit.base_minimum_reports_portrait_header"

The root cause is still somewhere else, though, since on another 7.0 installation also the default headers worked fine.