My land won't let me rez stuff, reports parcel full though I should have around 400 prims available

[Rolig Loon]

Maybe there's someone building on a sky platform.  Have you checked?  If you are the owner of the parcel, you should be able to open About Land and UNcheck the boxes on the Options tab that allow Everyone to build and to bring objects into your parcel.  Then return to the Objects tab and look at the list at the bottom of the page to see who owns things on the parcel besides you.  If you see an unwanted person's name there, select it and click Return to send that person's stuff back to them.

[Rolig Loon]

Maybe there's someone building on a sky platform.  Have you checked?  If you are the owner of the parcel, you should be able to open About Land and UNcheck the boxes on the Options tab that allow Everyone to build and to bring objects into your parcel.  Then return to the Objects tab and look at the list at the bottom of the page to see who owns things on the parcel besides you.  If you see an unwanted person's name there, select it and click Return to send that person's stuff back to them.

[Chic Aeon]

Rolig's advice about looking in the land tab to see what is wrong will likely solve your problem. It might not even be someone else's stuff. You may have dropped a necklace by mistake for example. You didn't plan for it to be there but it could be taking away a lot of prims.

Another less likely but possible explaination would be the change in land impact with the new land impact system. 31 prim boats can become 68 prim boats (not terribly practical) and things like that. Linking some prims that may not have been linked before (most often a partial mesh object or sculpt) can change the land impact a lot. I really don't understand the process well enough to tell you the "why" of it. There are limits on land impact on the types of prims. An old one prim sculpt can become (not sure how the become part fits in) 2 halving your actual usage. On the plus side a cube shape of any size is now only .5. Again, not likely this is your issue but something to keep in mind.

I watched a build I made go from 13 to 46 just by linking 12 mesh objects (1 LI a piece) to a hollowed cube. I am guessing it was the physics LOL, but I just unlinked and that worked fine.