The issue seems to be that the extension doesn't have permission to the domain used for getting the YouTube video info. The next update adds a check for it and will then prompt a permission prompt. The current workaround is to give it access to all sites: https://github.com/ajayyy/SponsorBlock/issues/611#issuecomment-801579094

Next update is still WIP, but that part is already implemented if you want to clone from source: https://github.com/ajayyy/SponsorBlock/pull/690