The Sony Ericsson Update Service (SEUS) updates your phones firmware. Part of the update that SEUS does is change the CID from 52 to 53. The CID is a form of protection which is maybe the cause of you not being able to mod your phone.
There is some a piece of software called JDFlasher that can flash your phone but to my knowledge no free software has been created to unlock or patch CID53 phones.