Problem with checkout.

[amusicsite]

Recently launched a new shop with prestashop 1.6.0.8

 

It has multiple payment currencies and two payment gateways, sagepay and paypal.

 

The problem is that sometime but not always, when a customer gets to the checkout there appears to be an extra quantity added to their basket, e.g. if you added 1x ItemA then when you get to the checkout there is 2X ItemA.

 

Also sometimes when you use the -/+ buttons to remove a product by pressing - till it dissapears then when you click through to the payment provider (either) you get the total before you reoved the items. Then when you hit back or cancel and go back to the checkout there are still the items you remove. When this happens then often the -/+ buttons do not work. 

 

Sometime but again not always you will get the error message "TECHNICAL ERROR: Unable to save update quantity in PrestaShop Cart" and can't update the cart.

 

Seems to be easier to replicate in Chrome browser on multiple platforms (XP, which a customer had when first diagnosed the problem then replicated it on W7 and OS X). Not managed to replicate it in FireFox but as it's an occasional bug then that is not a certainty.

 

Any help would be greatly appreciated.

 

 

The shop has the following modules.

 

Advertising block v0.9.2
Auto detect language and currency v1.3
Available quantities v1.4.1
Best-selling products v1.4.1
Best categories v1.4.1
Best customers v1.4.1
Best suppliers v1.3.1
Best vouchers v1.4.1
Browsers and operating systems v1.2.3
Carrier distribution v1.3.1
Cart block v1.5.7
Catalog evaluation v1.3.1
Catalog statistics v1.2.3
Categories block v2.8.7
Check VAT v1.6
Adds an addon to check VAT
CMS block v2.0.1
Contact block v1.3.3
Contact information block v1.1.2
Cron tasks manager v1.2.4
Currency block v0.3.2
Customer accounts v1.3.1
Data mining for statistics v1.4.1
Taxes & Invoicing
European VAT number v1.7.2
Facebook Like Box block v1.3.2
Featured products on the homepage v1.6.2
Google Analytics v2.1.1
Google Analytics API v1.0.4
Image slider for your homepage v1.4.5
Layered navigation block v2.0.7
Leo Bootstrap Megamenu v2.0
Leo Bootstrap Megamenu Support Leo Framework Version 2.0
Leo Manage Widget v2.0
Leo Slider Layer for your homepage. v1.0
Leo Theme Control Panel v2.0
Mail alerts v3.5.2
Manufacturers block v1.2.1
Merchant Expertise v1.10.0
New products block v1.9.2
Newsletter block v2.1.5
NVD3 Charts v1.4
Order reference change module v1.4
Pages not found v1.3.4
Payment logos block. v0.3.2
PayPal v3.8.2
Prestablog v2.3
Pretty URLs v1.3
Product details v1.3.1
Product payment logos block v1.3.6
Products category v1.6.3
Quick search block v1.5.2
Registered customer information v1.3.1
Sagepay v2.0.15
Sagepay Payment Module
Sales and orders v1.2.3
Search engine keywords v1.2.4
Send to a Friend module v1.7.2
Shop search v1.3.2
Simple HTML table display v1.2.2
Social networking block v1.1.5
Social sharing v1.2.9
Specials block v1.1.3
Stats Dashboard v1.3.5
Store locator block v1.2.1
Suppliers block v1.1.1
Tags block v1.2.3
Theme configurator v1.1.8
Top-sellers block v1.5.8
Top horizontal menu v2.0.9
User info block v0.3.1
Viewed products block v1.2.2
Visitors online v1.2.2
Visitors origin v1.3.1
Visits and Visitors v1.5.1

[Totti]

Same error with 1.6.1.0