Seeing a lot of "invest it in X" answers in this thread, but I'll echo what ForeverScrnd said — the real question is what you already know how to do. €85k isn't "start a business" money in most online verticals, it's "buy yourself 18 months of runway and test cheap" money.
I watched a friend do this exact dance a couple of years back. He had around €70k from selling a small content site and wanted to dump it all into one big idea. Instead he parked most of it, took €10k aside as what he called tuition money, and ran three small experiments: a niche affiliate play, a tiny lead-gen service, and a paid newsletter. Two of them died quietly. The lead-gen one was doing €2–3k a month within about five months, and only then did he start feeding it the rest of the capital.
That's the mindset I'd copy. If the idea needs the full €85k to work, it's too expensive — €5–10k should be enough to validate almost anything online. And keep six months of personal living costs ring-fenced before you touch a single ad account. Running on scared money is what makes people scale campaigns that were never ready.