So this started to happen just like that? With no obvious reason / update / config change etc.?

Also, do you have the program called Depends.exe? http://www.dependencywalker.com/
It would be useful to check if it is reporting any problems when run on the file “C:\Program Files\Microsoft ISA Server\avpxyftr.dll”.

Also, the event log is otherwise clean?

Thanks
Vlk