There is no code change between 9.1.502 and 9.2.
"sed: write error" is printed because /dev/kmsg refuse to been written with more than 1024 bytes at once (more than 4500 in your case).
The error is harmless, just as the fsck error. I see two options:
- Ignore the error and wait for 9.2.1
- Boot another Linux system (i.e. from USB stick) and use fsck.fat to fix the error on file system UUID=5DDB-A25D