*I* have no idea what would cause that error at that line nor why it would only occur
after "switch it to 15 min checking". Sorry.
BTW. The author of that plug-in is not the author of CalendarScript. There is also a note with the plug-in which states "Don't download this unless you can handle a few random bugs cropping up here and there." It was supplied by TubaDave whom doesn't seem to visit the forums any more. 
Dan O.
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