Outlook mail codeunit only works one time...

Hello,

I'm dealing with a strange problem. I use the codeunit Mail to sent email through Outlook. I want to open a window of Outlook with the message and an attachment.
The first time everything works fine, but when I run my code for the second time, I get an error on the following line

OAttachment := OAttachments.Add(BSTRConverterAttachFileName);

Then I get the error that there is no automation variable created. The codeunit is a singleinstance codeunit...

I use the NewMessage function to create a new message with the last parameter set to true, then I use the function AddBodyLine to prevent the message from the body text.

Does anyone knows the cause of this problem?