That is a developer website error, so all I can suggest (since I am getting it) is to keep trying until Apple fix the error on the website.
But it turns out there is a bug in the link, from a response to a similar question:
This URL works for me (browse to on iPhone):
https://developer.apple.com/services-account/download?path=/WWDC_2020/watchOS_7_beta_Configuration_Profile/watchOS_7_Beta_Profilemobileconfig.mobileconfig
xplora
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Didn't have an issue with the first beta, but now having an issue with the second, and not able to use safe mode either.