Login with another account

Hello, from a few days I have a singular problem with login on my ecommerce.

For 2 times it happened client A shop on my site, do the payment, client B shop on my site but don't buy anything
I receive the mail order with client A payment/shipping but with order's name and email of client B

I think I have a bug problem with any extension installed on my opencart.

I have the following extensions:
- Increase Page Speed 4.1
- Combo for Opencart 1.5.x
- Customizable Payment Method
- Customer Group Discount/Fee
- [VQMod] Live Search
- Multi Flat Rate Shipping
- Google Site Maps Generator - vQmod
- Exact Order Mail Copy to Admin
- Traduzione in italiano professionale (frontend e admin)
- FIX Search by tag + FIX Search by product category loses
- Html Everywhere

I don't know what could cause this

Anyone has an idea?

(1) You can make a new subdirectory, /vqmod/xml/testsetaside/, slip the .xml files into that (EXCEPT the open_cart one, which is required for vqmod to work), and see whether extension(s) give(s) rise to the error. You can also look at both /vqmod/logs/ and /system/logs/ when this sort of thing happens, in order to try to pinpoint it.

(2) Particularly these (being most of what you have):
- Combo for Opencart 1.5.x
- Customizable Payment Method
- Customer Group Discount/Fee
- Multi Flat Rate Shipping
- Exact Order Mail Copy to Admin
- Traduzione in italiano professionale (frontend e admin)
- FIX Search by tag + FIX Search by product category loses
- Html Everywhere