I've never had your particular error but it's worth a shot. When I have weird things happening with curves I often have to go into my historicals and delete weird historical parcels unintentionally created or historical parcels that have curve errors. Most importantly you need to make sure you can see ALL historicals. Not all historicals show up normally. Go to your historicals Definition Query and change it to just
SystemEndDate IS NOT NULL
Once you do that make sure there are no historicals underneath where you are working. I have found weird historicals and sometimes historicals with serious errors will screw up my curves. Once I delete them my curves again function properly.