Thank you for your help.
In my German operamail the icon is a wrench, and the option is named like "show text if possible" - I just didn't realize it yesterday.
Anyway, it's unticked and everything is fine now.
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I have just installed Opera 25, discoverd there is no more mailclient inside, so installed operamail.
Neither my old mails from good old opera nor new mails are shown in HTML format.
Also, there seems to be no way to activate it.