Thank you all!!!
Unreal forums are truly fast and helpful.
- unfortunately changing landscape coordinates does not help.
I managed to fix the problem by increasing the terrain resolution.
Still I will try what Riley1389 suggested because by using a higher resolution for the terrain I cannot increase the size of the landscape as much as I want. Before I used 144 tiles of 256x256, now (in order to be able to paint as I want) I had to increase the resolution to 2017x2017. So now I cannot use 144 tiles at 2017x2017 without performance issues.