From sdcc twitter:

After the successful badge sales for Special Edition and WonderCon using the same registration system, we have additionally tested continuously leading up to today’s event and each session has performed without issue.
San Diego Comic-Con
u/Comic_Con
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We, along with our registration system Configio, a Community Brands company (formerly Expo Logic/Epic), are investigating this morning’s issue and are determining the best efforts to move forward.
San Diego Comic-Con
u/Comic_Con
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At this point, the 9:00am start time is being delayed. We will have another announcement by 9:30am Pacific Time.
Thank you for your patience.