I don't know what is causing the palette problem, but here is a tip so you don't have to delete all those Slabs next time.
Basically you have to make sure each Slab has a unique ID. For this you can use the Document\Schedules and Lists\Element ID Manager menu command to set them.
Then you can again generate the 3D, the error message will come and with Find & Select you will be able to pinpoint the exact one Slab that is causing the error message. Then you can either modify its geometry to get rid of the error message or you can delete and recreate it, if that is the simpler way of handling it.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28