If you have only one file called "server.properties" then you could remove all the code that search for this file and write it directly.
Dim z As String
z = System.IO.Path.Combine(x, "server.properties")
Using t = New StreamWriter(z)
t.WriteLine(TextBox1.Text.ToString)
t.Flush()
End Using
Regarding the error, encapsulating the writing code with a proper using statement ensures a complete clean-up. Also adding a call to Flush() is probably not necessary, but doesn't hurt.