Hi,
could you post the FLL code of the engine? Are you using jfuzzylite version 5.0?
Meanwhile, could you try creating the rule proposition by proposition? That is,
ruleBlock.addRule(Rule.parse("if C2H2_C2H4 is A then DIAGNOSTICO is D1", engine));
ruleBlock.addRule(Rule.parse("if C2H2_C2H4 is A and CH4_H2 is B then DIAGNOSTICO is D1", engine));
ruleBlock.addRule(Rule.parse("if C2H2_C2H4 is A and CH4_H2 is B then DIAGNOSTICO is D1 and (C2H4_C2H6 is A)", engine));
ruleBlock.addRule(Rule.parse("if C2H2_C2H4 is A and CH4_H2 is B then DIAGNOSTICO is D1 and (C2H4_C2H6 is A or C2H4_C2H6 is B)", engine));
Once you know where it fails, make sure the variables and terms are written correctly. Once you find the error, I will revise how to provide a more meaningful description of the error.