Since the captcha was moves to runtime, the url to captcha called : plugins/cms/captcha became plugins/core/captcha

The problem is on a site, the url plugins/cms leads to a unknown plugin and was transmited to back ; but the url plugins/core leards to a known plugin and is handled by front

The problem is then whe the back try to solve captcha it do fail (because it is the front that knows it).

2 solutions I think:
1) redirect /plugins/core/captcha to back => front will never be able the check captcha, so the only way it could happens its for authentication that become impossible with captcha
2) recreates a cms helper for captcha that call the runtime one with a parameter to change the url , and add a pipeline (maybe authentified) that redirect to the core one