Most times when booting fails is because you do not have the kernels in /boot/, or do not have the correct modules for the kernels in /lib/modules/<kernel_id_string>/

In case of Nitdroid kernels, "standard" open mode kernels and l2fixed open mode kernels the modules are same as with the Nokia original PR1.3 kernel, so your modules are propably correct.

So, it is possible that the kernels are missing from /boot/ or are named differently than in ubiboot.conf file.

Just as peterleinchen says, look up the log file from /var/log/ubiboot.log, there will be cleartext indication of the selected kernel/OS, and if the loading succeeded or failed.