STM32F412/413 USB changes from STM32F411
The clock configuration for the F412 is significantly more complicated than that for the F411. If you refer to the F12 Reference Manual, page 112, you find Figure 13:

Compare that to the F411 Reference Manual, page 93, Figure 12:

So there are at least 2 extra bits which need to be correctly configured just to get the 48MHz clock enabled for the USB module.
In addition to this there are some new bits in the F412 USB registers, but (at least superficially) it appears that these shouldn't have any effect as long as they're left at their default/reset values.