Thank you. Now I can see what happens.
For some reason, some customers get their session cookie deleted by the browser. This can happen if they use Incognito mode in browser, but I doubt most of your customers do that. I couldn’t reproduce it in Incognito mode in Chrome browser though, the payment goes through just fine. The only method of reproducing this is to delete this session cookie in browser at the middle of the payment process (e.g. when you’re redirected to iDEAL confirmation page).
Anyway, I have created a new testing version of the core plugin. It has cookie lifetime increased to 7 days (it was previously set to “until browser is closed”). Can you please install it and let me know if this helps?