Unfortunately this is a known issue on Safari because it doesn't support certain headers that AWS uses. We're working with the Cloud9 team to resolve this asap though.
Please please please get the Cloud9 dev team to resolve this! There has been an issue open for over 3.5 years now [1] on the pre-AWS C9 and 1.5 years on the AWS support forums [2] and the main reaction from the C9 devs seems to alternate between "this is Apple's fault" and "wait what browser does iOS use again?".
Very frustrating that this is a web app and they can't make it work in one of the most-used browsers of all time. iOS13 even uses the full desktop Safari engine and it still won't load.