i am facing the issue from the last one week and the agent is not aware of the issue as well. ( You are not signed in Error 403 )
it happens occasionally. i myself wasted a lot of time on it. even if you logged in you can be logged out anytime and see this 403. it's a very unstable system but the best PGP applicants can have. Try use the new online thing for spousal sponsorship.
the easiest way if you are not in a rush, wait several days and try again.
something you can try if you cannot wait:
1. clear your browser's cache/cookies
2. change to incognito mode or a different browser
3. change a different OS/computer
4. change network/IP (eg. cell data instead of wifi).
in that sequence.