You must also copy modules from kernel-cssu-modules_2.6.28-10cssu3_armel.deb package to maemo rootfs.
Afair u-boot had (has?) the stock kernel attached. So you should have only selected run attached kernel!?
Now you have cssu kernel and possibly no modules, as said. I would go and flash the u-boot kernel (package u-boot-flasher) if its size is around 2MB. Then you should be able to run attached kernel.
# ./0xFFFF/src/0xFFFF-i386 -m u-boot-2013.04-1+zImage-2.6.28-omap1.fiasco -f 0xFFFF v0.6 // Open Free Fiasco Firmware Flasher Warning: Removing unknown image (specified by file u-boot-2013.04-1+zImage-2.6.28-omap1.fiasco)
# ./0xFFFF/src/0xFFFF-i386 -I 0xFFFF v0.6 // Open Free Fiasco Firmware Flasher Not a local device Waiting for USB device... Found USB device: SU-18/RX-44/RX-48/RX-51 (0x421:0x105) in NOLO mode USB device product string: Nokia N900 (Update mode) USB device serial number string: MUM364446 Detected USB device: RX-51 Initializing NOLO... Device: RX-51 HW revision: 2101 NOLO version: 1.4.14 Kernel version: (not detected) Initfs version: (not detected) Software release version: RX-51_2009SE_21.2011.38-1_PR_MR0 Content eMMC version: RX-51_2009SE_1.2009.41-1.DACH Root device: flash USB host mode: disabled R&D mode: disabled
If all fails, just extract stock kernel and load/flash this one to device, as you have done with cssu. This should boot.
Maybe you were running older version?