I managed to get around it as follows, but I'd say it is a bug:

What I wanted to do:

case
when ((p.StartDate <= cast('Today' as date)) and (p.EndDate >= cast('Today' as date))) then
(cast('Today' as date) + 1)
else
p.enddate + 1
end

What I ended up doing to get around the problem:

case
when ((p.StartDate <= cast('Today' as date)) and (p.EndDate >= cast('Today' as date))) then
(cast('Today' as date) + 1)
--else problem with dates and else section
when (p.StartDate > cast('Today' as date)) then
p.enddate + 1
when (p.EndDate < cast('Today' as date)) then
p.enddate + 1
end