I haven’t been able to reproduce it at my end (Firefox), but a user (Chrome) just sent us this screenshot and told us that the captcha wasn’t working for them.
Event application page: Open Science Retreat 2026 (7-11 April 2026): Registration · DRA event platform (Indico)
Potentially related to this issue: 422 Error on abstract submission (not that anything specific suggests to me that they’re related - just that I’m not an expert and we’re experiencing both simultanously)