As the upload size is limited to 500mb, and my local savefile was beyond 2gb, I had to delete chunks with MCAselector.
Now, we like to explore and make map walls, and here we had a giant 6-by-10 level 3 map wall.
After deleting the chunks, opening the world locally has no effect on the maps, but if I upload the savefile, the maps are either missing or corrupted.
I tried re-uploading the map_###.dat files, but it did not really solve the problem.
Is there a way to preserve those map walls or should I give up and be ready to re-explore everything?