styrax,
yes , your split of the 2 phases is fine.
in preparation phase, the error may come from:
1/ congestion on 2G cell
2/ misconfiguration of 2G cell in the 3G network
3/ misconfiguration of handover parameters between 3G and 2G (for example, the 2G might be forced to reply to 3G with the message “I’m overloaded, don’t send any UE from 3G here !…)
in execution phase:
1/ real radio failure : the UE is in such a poor 3G radio that it can’t decode the relocation command
2/ wrong BCCH/BSIC provided to the UE
3/ wrong TBF information from the 2G (or whatever info the 2G sends to the 3G BSS regarding what’s waiting for the UE in the 2G cell)
Further testing would be necessary : I never tried it myself, I’m just thinking out loud…