When posting details from Sage Business Tax to Sage Partnership Tax Box 3.7 on Page 2 populates with the Partnership start date even if it is not in the current tax year. The return will not validate and has to be copied and the date removed.
This does not happen on all Partnerships. The one we have had today started as a sole trade in 2008 then become a partnership in 2012. The 2012 date went into the 2018 SA800 return and caused the issue. So may only affect businesses like this.