Many thanks to Kika for a great help! Without her, it would be unsolvable for me for I would never expect such strange shapes of the dots...
SPOILER: Diagonals from => top => to r5/c1 => r1/c5 => r4/c8 r8/c1 => r4/c5 => r6/c7 r11/c1 => r7/c5 => r8/c6 r14/c1 => r10/c5 The right side is symmetric to the left one...
Thanks Joe, I had trouble understanding your hint though, but they inspire me to get it. To either side of the tree there are four lines going up parallel to the tree. The lines start at r1/c5,8,11,14 and continue up 5 squares in total. Top two lines then go back down diagonal first three, then two, then one, then no squares. Sort of like holly or something hanging behind the tree. Nice picture except for the impossibleness of it but I can see why the designer thought it might be solvable.
SPOILER: Diagonals from => top => to
r5/c1 => r1/c5 => r4/c8
r8/c1 => r4/c5 => r6/c7
r11/c1 => r7/c5 => r8/c6
r14/c1 => r10/c5
The right side is symmetric to the left one...