This is a unknown issue with Certificates in BizTalk Server. You see the error every time when you open or save send port configuration, it won't affect the functionality, but very annoying. It usually happens when you remotedly log on to a BizTalk server at which the "Other People Certificate Store" is not initialized.
Microsoft has an article talking about this issue, and it suggests to initialized the "Other People Certificate Store" during the Group configuration. However, if you have your BizTalk group configured already and don't want to reconfigure it, here is what you can do,
1. Open "Internet Options" from control panel, select "Content" tab, and click "Certificates".
2. Click "Other People" tab, click "Import...", this will launch the import wizard. Click "Next" then Click "Cancel", because you don't really have a certificate to import.
3. You should go back to "Other People" tab now. Click "Advanced" button, check all check boxes, click ok to save it.
Good luck!
...Edmund Zhao