trigger for cb subs promotion

Your custom code is saying the code doesn't exist or CBSubs is saying the promotion code doesn't exist? Please ensure you've CB 2.x nightly and CBSubs 4.x nightly to ensure you're up to date.

CB 2.x Nightly:
www.joomlapolis.com/downloads/doc_details/640-community-builder-nightly-build

CBSubs 4.x Nightly:
www.joomlapolis.com/downloads/doc_details/671-cbsubs-gpl-nightly-build

this is CBSubs that tells that the code doesn't exist (despite the fact the code exists)

and, on the same page, I try a second time to enter the code, and at the second try, it works fine..

I don't get nightly version you re talking about because it s been said :
"It is intended for developers and testers and should not be used on live sites unless thoroughly tested first."

Is this for purchase during registration or from your Subscriptions tab? I wasn't able to confirm your issue. My coupon code applied on the first attempt. Please upgrade to the nightly releases linked above and see if your issue persists.