Uh oh... this is the same corner as in the 11th of November picture. Today I was inspecting more closely, and found out that in addition to holes in the very corner, there were lot of bad parts in one of the logs. Not just any log, but the very log which was supposed to hold the weight of the ceiling... Using the tip of a knife I could just wipe away part of the log - it turned into dust falling on the floor.

So, in the picture we see both walls and the ceiling, and then a horizontal post which supports the ceiling. And where that horizontal post meets the other wall, instead of resting on a solid log, it now hangs in the thin air. I installed a make-shift vertical post (it is just to the left, bit outside the picture - on the floor there is a bottle jack, on top of that there is a vertical post which connects with the horizontal post. Using the bottle jack I could lift the ceiling one centimeter. Now I only need to figure out how to fix this (I already have some ideas, but better think about them for a while to sort out a good solution)

And then an interesting detail - I think the ceiling is as it was built in 1905. The ceiling is made of split logs, and there is dried moss as insulation material. You can see some of the moss (that dark spongy material) in between the ceiling planks.