Prolonged wet period could do it. According to John Hunter's "The Fate of Africa", the scramble for Africa succeeded because the continent had just entered a period of drought. Keep the climate wet for a little longer, and the extra population/food security could give Africans a military edge against occupation.
So with a POD after 1838 find a way for the colonization of Africa to have failed to gain much ground there and for much of the continent to remain independent of European rule.
I think your best bet is to have some early venture be a horrible expensive catastrophe similar to the British Afghanistan campaign, to the extent that no other government wants to repeat it.
Its not a question of failure. Europeans weren't chomping at the bit throughout history to conquer Africa.
However...it is perfectly possible that they might not bother.
Get rid of the Franco-Prussian war and the subsequent economic crisis and depression and you may not see Europeans frantically carving up Africa to try and spur growth and head off others in the now turbulant economic situation.
I'd have to think though that eventually European business interests would start pushing into the interior...which would not be pleasant; imperialism is a moderating force on capitalism. Take away the imperialism and you end up with bodies like the Congo Free State.
About the latest PODs where this is feasible are either averting the Anglo-Egyptian War or alternately Leopold II's bid to take over the entirety of the Congo region. Leopold had the conscience of Reinhard Heydrich with balls of adamantium, but his gamble was warned against by a lot of people. If the Scramble's two direct causes are either singly or jointly averted, there's rather less cause on the part of the rest of Europe to want Africa without Belgium grabbing a portion the size of all of Western Europe, meaning everyone else wants it too, or the UK deciding that indirect rule was not enough, direct rule was preferable.