EDIT: Sorry just reread your post and noticed you said you were using prefabs. Do you mean the ones that came with 3dgs? If so then your best bet would probally just try building them yourself... Im not sure why they would produce the error as Id figure the prefabs would be properly built... never used them myself tho so couldnt really say.

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Delete the block. You made it too thin... Did you use CGS subtract? If yes thats probally where it came from. You shouldnt use cgs subtract but instead build the faces by hand using face edge and vertex move.

You can find the block easily by going too edit> find object and typing the number of the block they gave you in the error.. You may have to do it a few times to find all the blocks tho.