A CRC error during an update is not common in this case and indicates a communication problem that is not related to the firmware version, because only the bootloader is running at that time and it is fixed from the factory.
In most of these cases, the USB to RS232 adapter is to blame, there are often problems with the drivers, for example when migrating to a newer version of Windows.
I would recommend testing the device using a different power supply, a different PC and a different converter.
You don't mention whether the encoder is producing RDS data and whether the LEDs are working. This would help narrow down the possible causes.
For completeness, I will refer you to page 58 of the manual, where steps are generally described in case communication cannot be established (for example, if addressing or No-Header mode has been activated):
https://pira.cz/rds/manual.pdfOf course, we can still test your board, or restore it to its original operating condition, but I believe that this will not be necessary.