Can't log in my paypal account during transaction

Hi,
I've just installed your Paypal plugin on a fresh Litecart install.
Then, I tried to buy a product on my Litecard website, but when I'm redirected to Paypal's page and asked to insert my credentials Paypal won't recognise my user ID nor my password.  If I connect, instead, directly to Paypal without purchasing any good on my ecommerce, I can log in without problems.
Any Idea?
Thank you
Kind regards
Nicola
I think, it is not related to add-on. 
Please reset your browser cache or try in different browser.

Regards
Are you trying as testing mode ? (Sandbox mode) 

Change to production mode in backend. 

Please send website URL.
Hi,

I've just disabled "Test Payment" and  made an attempt with another browser but the problem still remain.

Maybe it's related to Sandbox mode, as you suggest (while attempting my purchase I'm redirected to https://www.sandbox.paypal.com/)... but I don't know where I can change to Live mode (sorry, it seems to be a silly question)

Regards

N.
Your screenshot is configuration is correct. Which version is using ?
Please send cpanel login details to support@ciphercoin.com then I can check it.
Hi,

I'm using Litecart LiteCart 2.1.4.

I've just sent you my cPanel login details

Regards

Nicola
Please send admin login details also. 

Note: your email is not working, while replying to you.
Hi,

just sent by mail

Regards

Nicola
In admin, I just disabled to testing mode then it is working fine now. Please check it.
Hi,

thank you for your help.
I made a test: everything works well, but after the purchase, when I press "redirect" button on Paypal's site (redirect, for now, is not automatic in may Paypal's account configuration), i get this message in my e-commerce page: 
"There was an error verifying your transaction"
Any idea?
Kind regards
Your ext folder was 777 permission. Rest cache and try again.
Problem solved: works perfectly!

Thank you 
Best regards
Nicola