BobD

I'm personally not a fan of Fujinon lenses in general, zoom lenses in particular although there are a few good (expensive) ones out there and this image does not look sharp to me. .No offence intended - just my opinion. Any teleconverter will also degrade sharpness and your image is a good example of this. I use a Canon EF400 f5.6 (on my X-T4) with the K&F EF-FX adapter at a total combined cost of approximately £600 GBP and obtain results better than this..pse see attached.

Hi all!
apologies for my late reply. I'm using the XT-5 with the latest firmware. I also tried to update the flash, but that's not possible. I'm using a MacBook and somehow that doesn't really work. Anyway, I tried everything.. including switching on the camera first, then turning on the flash. Nothing changes. 
But I am using rechargeable batteries (fully charged) - maybe it might be worth trying different ones (need to buy some first). Will keep you updated! 🙂
 

The adapter has arrived and I've been testing it on the X-T4 with the EF 400 f5.6 and so far I can't really fault it. Autofocus speed and accuracy is excellent, albeit tested only on single point focus, which is what I use 95% of the time. I can't comment on face or eye AF as I haven't tried it and I'm not a big fan of either anyway - just not features that I use. So far the lens is 'fixed' on its widest aperture of f5.6 but that has not been an issue for me at all so far and I have noticed during experimenting with camera settings that it has stopped down by itself for reasons that I am unsure about to be honest however it's very early days so it may be that there is potential to be able to alter the aperture on demand if I actually want or need to. I have yet to find any issues to make me wish I had gone for a considerably more expensive adapter with aperture ring. Attached jpeg, which has been through Topaz Photo AI and cropped, shows what this combination of lens and adapter can do.

Welcome to the forum.
That is a really good lens, hopefully it will give you many great images.
You did not mention which K&F adapter are getting, they make several, but not all of them have the aperture control ring on them. Since that lens does not have an aperture ring, the price for the adapter with the ring (if you want it) is not much more than for the one without the ring.
I am mentioning this so that if the menu does not allow you to change f-stops for that lens - adapter combination (the lens will stay wide open for every shot), it is still possible to get it to work.
Best wishes.

I've ordered the AF version without aperture ring as I found it at a very good price (new). I'm reasonably confident I can make aperture changes in the camera menu however if that turns out to not be the case I'll probably sell the adapter and get the other version, or possibly the Fringer, with the ring. TBH with the weather/light conditions we've had in the UK for some time now I'll probably find shooting at a constant f5.6 obligatory.