Hello
I have exactly this problem. I'm still in the test environment of Postfinance. the payment works, just the return to hikaschop make a blank page.
I look at the apache error log, but no errors are showing just warnings.
the return address is which makes a blank page :
http://tuenni.ch/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=postfinance&tmpl=component&lang=de&Itemid=393&orderID=95¤cy=CHF&amount=2&PM=CreditCard&ACCEPTANCE=test123&STATUS=5&CARDNO=XXXXXXXXXXXX5162&ED=0218&CN=Pierre+Burr-Wittke&TRXDATE=07%2F27%2F15&PAYID=44727477&NCERROR=0&BRAND=MasterCard&IPCTY=DE&CCCTY=DE&ECI=5&CVCCheck=NO&AAVCheck=NO&VC=NO&IP=84.175.76.139&SHASIGN=C2D68602801733D78BB6FC4EA1433B73DA59D493
in the log of Postfinance it only says, "PostFinance FEHLER währen DER POSTSALE-ABWICKLUNG" -> PostFinance error during POST SALE processing.
I enter the SHA-IN signature & SHA-OUT signature in the Postfinance backend and in the Hikasho Postfinance plugin.
I have hikashop business 2.5.0, Joomla! 3.4.3, PHP 5.4.35 % Firefox 39.0.
Error reporting on Joomla to maximum.