Same issue - plus the update box is no longer stating that my Macbook Pro is registered for beta software. I've reinstalled the beta profile, which takes me to the app store displaying Big Sur as a download.
However on a different thread the following procedure fixed the issue for me and beta 4 is downloading now:
open terminal
sudo -i
enter root password
copy and paste this to command line:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil
Unenroll by copying and pasting this to command line:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil unenroll
Enroll in the DeveloperSeed Beta by copying and pasting this:
/System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed
Close and re-open System Preferences > Software Update
Give it a bit of time.
The new Beta should now show up.
Thanks to @Steffen_Voglmann ^^
mdshamilton
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I have the same issue - I'm having to use Chrome as I cannot figure out how to get Safari to work again. Hoping that Beta 4 fixes this issue.