I had the same error. I did a rpm-ostree reset and afterwards: rpm-ostree rebase fedora:fedora/33/x86_64/silverblue and now I see in status the new base:
$ rpm-ostree status
State: idle
Deployments:
ostree://fedora:fedora/33/x86_64/silverblue
Version: 33.20200917.n.0 (2020-09-17T08:03:28Z)
Commit: 2e286e6187204c900eba6d123b3d385383c4d1ab0788afbdc6b18a27546374fc
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
Diff: 1090 upgraded, 79 downgraded, 38 removed, 63 added
● ostree://fedora:fedora/32/x86_64/silverblue
Version: 32.20200917.0 (2020-09-17T15:45:15Z)
Commit: 55f1a53590df434f652a3e628d51ef222dab007c506cf60713b0c77491f15d28
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
ostree://fedora:fedora/32/x86_64/silverblue
Version: 32.20200917.0 (2020-09-17T15:45:15Z)
BaseCommit: 55f1a53590df434f652a3e628d51ef222dab007c506cf60713b0c77491f15d28
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
LayeredPackages: fedora-workstation-repositories
ostree://fedora:fedora/32/x86_64/silverblue
Version: 32.20200905.0 (2020-09-05T18:11:38Z)
BaseCommit: 4150be4c9953a25b2ce8dc4197df6e2221014d23d8a162550dc0391ba13d692b
GPGSignature: Valid signature by 97A1AE57C3A2372CCA3A4ABA6C13026D12C944D0
LayeredPackages: fedora-workstation-repositories
Pinned: yes
**Unfortunately my F33 is not starting. I only see the small Fedora symbol at the bottom and then **
the boot process stops.