@Sven, I think you are confused, this report it was not about a captcha type, it was about "Use only if PR" range in custom captcha setup inside options for a project. I found out that if range starts from 0, then GSA disables the checkbox, probably is parsing it as a false boolean and not as a range starting with 0.
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OK now I see what you mean. My previous comments have been attached to the wrong threat. I have deleted them now to not confuse anyone else. Sorry.