For anyone facing the dreaded app catalog sync failures, I investigated this yesterday (cf. TrueNAS Apps catalog fails to sync - Git HTTP/2 returns 401, HTTP/1.1 works - #9 by csj )
But this is confirmed to be an upstream GitHub issue (cf. TrueNAS Apps catalog fails to sync - Git HTTP/2 returns 401, HTTP/1.1 works - #26 by csj )