Open up your mini and write down exactly what RAM is in what slots by manufacturer like shown in your screen capture. You may be using incorrect RAM or assuming that there is more RAM installed than what you think.
I will, apologies. Though I was saving time by looking into the receipts rather than opening it up.. Need the mac mini for work tomorrow morning so can't open it now but will try tomorrow evening CET in case you haven't lost patience 🙂
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