Not sure...
Note: Possibly set
ALLOW_PERSONAL_INVENTORY_OVER_CAPACITY = true //Personal Inventory can go over
Then go over capacity, give yourself some slabs and check.
I am using my own server, my own texture pack, my own custom blocks. I have edited the StarMade-dedicated-server-windows.bat file and added the -d64 as copy and pasted:
java -d64 -Xms256m -Xmx8192m -Xincgc -Xshare:off -jar StarMade.jar -server
so it uses 64bit java, I also made a custom loading screen and I am using Kupu's blurry fix.