You are using the wrong overload (this one is expecting a Char
not a String
). The one you are looking for is:
Dim SplitRTFmessage As String() = myRichTextboxEx.Rtf.Split(New String() {"<CUTHERE>"}, StringSplitOptions.None)
This is the right way (one of the alternatives you have) to use a string as a separator and thus delivers the right result (2 parts).