Rv-on-web & Isabel authentication cards: “The page cannot be displayed”

Today I came across an issue where a customer could not login to Rv-on-web (Belgian withholding tax administration website) using an Isabel card. After countlessly checking the certificate chain, I tried to log in to the Isabel webservices, and that didn’t work either. But I was redirected to a troubleshooting site which led me to this article: Error: “The Isabel 6 software cannot be accessed with Internet Explorer 64 bit” when trying to access Isabel 6. After following the instructions step 2 (setting TabProcGrowth in the registry), the magic pincode entering applet finally appeared and login to Rv-on-web was working.

This is the registry change in question:

Windows Registry Editor Version 5.00 
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main]

More information on TabProcGrowth: Opening a New Tab may launch a New Process with Internet Explorer 8.0 (also applies to Internet Explorer 11). I suspect the pincode applet fails to load when its started in a new process after clicking the “login” button on Rv-on-web, leading to “The page cannot be displayed”, because the selected certificate cannot be decoded without the pincode. Setting TabProcGrowth to 1 changes this interaction. It’s not immediately clear to me why this trick works though.