Problems with shpiping cart and stock by attributes 1-4-14

Hello everyone!
I have a problem regarding a ""Whoops!-Sorry-but-you-are-not-allowed-to-perform-the-action-requested". I did not understand why, but 95% of my clients received this message and they cannot buy!
Looking at the forum I understood that the problem is in stock by attributes 1-4-14 includes/modules/pages/checkout_shipping/header.php.
If i delete it and I return to the v13.9f version, all is working fine in the site, but stock by attributes do not work!
How to fix this problem?
To explain better,if a client register and then buy immediately, there is no problem, but if a client return to the site or choose something to buy and THEN register or enter, they receive the blocking message.
Help me, please!

Did you merge all files correctly when you installed the mod?

picandnix:

Did you merge all files correctly when you installed the mod?

Yes, I checked all files.