There is nothing wrong with the firmware. This is a bug of my tool. I haven't corrected it because DB2020 CID49 phones cannot be patched for free, so there is no real use.

First you need to upgrade the EROM CID to 52: connect the phone to XS++ v3.1 and when it ask you if you want to upgrade the CID, press the YES button.

Download a CID52 firmware(ex. K800_R1ED001_MAIN_GENERIC_WI_RED52.mbn), and follow the tutorial in first post of this thread(the DB2020 CID52 section).