Hi,
I am sorry but it looks like the problem comes from iDeal.
I suppose that they make an "ob_get_clean" without doing an "ob_start" after that. It breaks the "output buffering" for the rest of Joomla so it display the HikaShop content where it should be displayed.
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.