Thank you for the very quick reply and the hints. I did as you suggested and compared the file with the one you provided in the link – they were exactly the same. So, luckily the file on my machine was unchanged. Therefore, I guess, ImunifyAV gave me a false alarm.
Anyway, I will install version 3.11 now – so the alarm should disappear because the polyfill.js file is not available anymore then.
Thank you again for your help! Very good job! I really appreciate it.