Just an idea to add to previous solution, using interpolated string could make it simpler to spot these kinds of problems. In other words you could use
workbook.Save($"{My.Application.Info.DirectoryPath}\Reisdocumenten\{Form1.Reisnr.Text}.xlsx")
Using this format the Visual Studio quite nicely shows with color coding which part is literal and which is not.
For more information, see
$ (Visual Basic Reference) | Microsoft Docs[
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