Hi,
As per the error information you provided, this error happens in the Joomla plugins manager when you edit the iDEAL plugin settings.
This issue is between Joomla and the plugin.
Updating HikaShop or not won't change anything about this and there is nothing HikaShop can do about it.
You'll have to contact the iDEAL plugin developer about it.
I was talking about the part "Under Configure payment methods I get a white screen.". I understand you're talking about a white screen when you access the menu System>Payment methods of HikaShop. So please activate the debug setting and check on that page.
Now, it's possible you get the same error and in that case, the solution will be the same: fix the iDEAL plugin. But maybe you'll get another error and we can help there.