Error #23904 after using the latest wiimmfi patcher on Dolphin

Can you tell me the serial number of the WiiU your NAND is from, so that I can check the server logs?

No, if Dolphin is set up correctly then Wiimmfi won't even know / notice that you are using one NAND on multiple Dolphins.

Your error is caused by a bug in whatever tool you used to create your NAND dump. You can use this tutorial - Fix for error 23934 on Dolphin - to fix this (even though that tutorial says 23934 and you get 23904).