The most common reason for that particular error that we see with our software is that people enter the 2 week's absences as Mon-Fri. ie Monday of one week to Friday the following week.
SPP (and indeed SMP and SAP) take no notice of working days. You should enter the SPP absence as full weeks, ie Mon-Sun. (or Tue-Mon, Wed-Tue, and so on).
Obviously I can't comment about Sage, but my guess is that it may be easy to make the same misunderstanding.