I have a problem

The landmass of my mod dissappears. Every time I enter the generated cells, the landmass/ land mesh dissappears, except for a few cells. I'm falling into the water and I see floating trees.
The CS shows all and doesn't give problems, except a few warnings: Trying to get exterior cell for invalid cell coordinate. Values must be between -32768 and 32767.
Could not find cell (0, -524288) in world 'Wilderness' (0102F84E) to add reference 'TreeWhitePineYoung' (0102F84F) to.
Reference attached to wrong cell for its location:
REFR Form '' (0102F84F) to TREE form 'TreeWhitePineYoung' (00000B7E) in WorldSpace '...' (01000ED3)

Can anybody help me?

okay, that error has nothing to do with your problem....one of the objects you have placed has ended up in some crazy far away cell....ive had that before, never found how to fix it.

it might be that the cells arent there cause they are empty....try putting a tree in each on in the CS and see if they show up then.

I've had that error as well. I believe I corrected it by deleting the reference in the Data->your esp->show details window iirc. [Edit: I actually had to use TES4Edit to remove these. Deleting them from the CS didn't work for me.]

As far as the missing land goes, if you created the cells in the heightmap editor and neglected to correct the error that it generates in the corner of each quad you can get this effect of the land disappearing. If this is the case, just go to the corner of each quad (cell 0,0 cell 0,31 etc.) and flatten out the hole/spike. This has corrected it for me in the past.