This morning Fujifilm announced a short press release to warn its customers about a problem involving the latest firmware update available for the X-T1 (version 4.20) that was released just 5 days ago. Quoting their statement, they say:
We have found a malfunction in the firmware version 4.20 launched on December 17 2015, which rarely causes freeze when shooting in AF-C mode.
Fujifilm is suspending the download for the new firmware. For customers who have already upgraded their camera, there is a new firmware version 4.21 that is essentially a copy of the previous 4.10 version, meaning it will downgrade the camera’s firmware to the version prior to the buggy 4.20.
If you upgraded your X-T1, we suggest that you update it to firmware 4.21 to avoid malfunction. If you still have a firmware version prior to 4.20, you don’t need to do anything. To check your firmware version on the camera, turn it on while holding down the DISP/BACK button.
If you need to upgrade to version 4.21, you can follow the instruction at this link here. This applies to both the Fujifilm X-T1 and X-T1 Graphite Silver.
Fujifilm will release a new firmware in January 2016 that will fix the problem.