Greetings, Razer, I wouldn't say the meanings are completely different. In fact, one of the meanings of 'serious' is ' sincere about sth; not joking or meant as a joke ' (taken from Oxford Advanced Learner's Dictionary ). Given that 'sincere' is understood as ' free from pretence or deceit ', 'serious' is the milder alternative in your case, somewhat euphemistic, I would even say.
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