what is wrong???

create function SvensktDatum2(@indate datetime, @separator char(1))
returns nchar(20)
as
begin
return
convert(nvarchar(20), datepart(yyyy, @indate))
+ @separator
declare @manad nvarchar(20)
if (datepart(mm, @indate)between 1 and 6)
begin
set @manad ='0'+ (datepart(mm, @indate))
+ convert(nvarchar(20), @manad)
end
else
begin
set @manad = (datepart(mm, @indate))
+ convert(nvarchar(20), @manad)
end

+ @separator

declare @dag nvarchar(20)
if (datepart(dd, @indate)between 1 and 6)
begin
set @dag ='0'+ (datepart(dd, @indate))
+ convert(nvarchar(20), @dag)
end
else
begin
set @dag = (datepart(dd, @indate))
+ convert(nvarchar(20), @dag)
end
end

I get this message:

Server: Msg 170, Level 15, State 1, Procedure SvensktDatum2, Line 20
Line 20: Incorrect syntax near '+'.

The error message seems pretty clear to me.. See line 20 of your stored proc where it says:
+ @separator

That section seems to be by itself and is not part of any expression or anything .

BTW are you a Swedish speaker by any chance?

Well now, I've solved my problem and yes I am a Swedish speaker!!!