Date Convertion Problem

I had the following user defined function working on a test server.

CREATE FUNCTION dbo.GetFirstDayOfWeek (@WeekNo Integer, @YearNo VarChar(4))
RETURNS DateTime AS
BEGIN
DECLARE @Date VarChar(10)
DECLARE @FirstDayOfWeek DateTime

SET @Date = '01/01' + @YearNo
SET @Date = CONVERT(VarChar(10), CONVERT(DateTime, @Date,101) - (DATEPART(DW, @Date)-1),101)
SET @FirstDayOfWeek = DateAdd(WW, @WeekNo-1, @Date)

RETURN @FirstDayOfWeek

END

It should return the date of the first day in the week when provided with a week number and year, however, the test server has been rebuilt and after transferring my database over, this function no longer works. I now get the error message "Syntax error converting DateTime from character string".

The database is exactly how it was on the previous test server build - could my problem be caused by the server being setup differently (i.e. different regional settings and therefore different date format) or am I completely missing something in the SQL?

Unfortunately I do not have enough rights on the server to check the SQL Server settings!