Upgrade creates database issues

-- url of the page with the problem -- : mywebsite.com/mypage
-- HikaShop version -- : 2.3
-- Joomla version -- : 3.1.6
-- PHP version -- : 5.3.28
-- Browser(s) name and version -- : XXXXX x.x.x
-- Error-message(debug-mod must be tuned on) -- : Error_message

When trying to upgrade to Hikashop essential: enter shop, click on a product and it redirects to site Home Page. rather than to the product page.
In Admin, Hikashop>products>product folder, select a product and it shows an SQL database error.
Have done a backup restore to reinstate the database and free version of Hikashop.
Have spent several hours trying to track down information on this but don't seem to be able to find any reference to this issue.
Thanks.

Further to this above problem - I have access to the write function of the database, so this is not an issue. Have again tried to upgrade and again, it breaks...
Here is the error message displayed against products after upgrade:

An error has occurred.

0 SQL=SELECT b.*,c.* FROM gf_hikashop_category AS a LEFT JOIN gf_hikashop_taxation AS b ON a.category_namekey=b.category_namekey LEFT JOIN gf_hikashop_tax AS c ON b.tax_namekey=c.tax_namekey WHERE a.category_id = 11 AND b.taxation_published=1 AND ( taxation_type = '' OR taxation_type = 'company_without_vat_number' ) AND b.taxation_date_start <= 1392610655 AND (b.taxation_date_end = 0 OR b.taxation_date_end > 1392610655)

Any suggestions...

Cheers,
Keith