Hi,
For the Paypal adaptive plugin, in sandbox mode, the official sandbox application is used : APP-80W284485P519543T
But if you got the API credential incorrect message, you should check your user/password/signature.
Please check that you do not add extra character (like space) in their.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.