At this point my best guess is:

Your email hosting service provides a folder called "SentMail". This is common for some email hosting services as the default place for sent messages. But (for some unknown reason) Outlook can not access it.

I really don't have any other suggestions except:

8 -- Shutdown Outlook and delete the .srs file
What is the SRS file? (And where is it?) - Outlook Tips

11 -- Create a brand new Outlook Profile. And set it allow you to choose which profile to use when starting Outlook. Add you email account to the new Profile. Test with the new profile. If it doesn't fix it, go back to your original profile.
Create an Outlook profile - Microsoft Support