Well I guess you were using MeGUI supplied version of Avisynth+, and its plugin directory.
ImageSeq.dll, ie ImageSource is a separate plugin in AVS+, not builtin as in older v2.60.
Not sure whatis happening now.
Post your script again, and are you using megui to view ?
What happens if you view script in VDub2 ?
Also try
whats it say.