I think it is some obscure threading problem as your code seems to work fine when optimised compiled and and my demo worked in the IDE when I was testing.
The IDE error implies that textbox1.text is null (which is not the case) however if you continue after the error it now works OK. It also works if you copy the RTF first! If I legacy compile both my demo and your code I get this error :
"Current thread must be set to single thread apartment mode before OLE calls can be made. Ensure that your Main function has STA ThreadAttribute marked on it."
This is required as the clipboard in .NET calls the native clipboard code which is a COM object which needs an STA host. However Erel has told me that Basic4ppc IS now STA threaded (which is why I couldn't produce this library and the WebBrowser before as both are COM objects needing STA) so I have no idea why it is throwing this error.
Perhaps Erel can shed some light on this if he reads this!
The RichTextTextbox has a workaround for this threading problem, which I thought had now gone away with the change in threading for B4ppc so I will try to to produce a version of the clipboard library incorporating this workaround.