The “Caused by” seems to be the same issue we face here: https://community.sonarsource.com/t/bitbucket-pipelines-started-failing-could-not-find-a-default-branch-to-fall-back-on/65144
Same story - 2 different projects broke at the same time over the weekend (so no code changes we know of), same error.
@ganncamp we use Bitbucket as ALM though