Why is checking out an excel document programmatically from SharePoint not working

Question

I'm trying to check out a document from SharePoint from VSTO. But it keeps nagging that the document cannot be checked out.

I'm able to check out the document manually, using the same credentials.

What I already saw is that if I turn off the "Require Check Out", that I can perform a check out on the document programmatically. Unfortunately there is not an option for the end-user.