I've had a similar issue a couple of times (years ago) with an X-T2 and it happened again with a loaner GFX50S a while ago. Usually it was solved by switching the camera off/on and in between re-mounting the lens. I've talked about this to a camera repair company here in Switzerland and this is the (unconfirmed) story they gave me:
>>The older X-cameras don't have a particularly strong mount. That is why Fuji made a big point about strengthening the mount of the X-H1. They told us it was for the upcoming heavier 'pro' lenses, but in fact it was for all lenses beyond the smaller f2-line. When the contacts between camera and lens don't work well, two things may happen: (a) no contact, which results in the infamous "f0" error message or (b) poor contact which leads to an electric charge building up between the contacts that will trigger eventually the "turn off/on" message to force a kind of discharge. Forcing comms between camera and lens (like a firmware check, induces a similar reset. The contacts of both the X and the GFX mount are quite fragile, but in order to maintain compatibility there's not a lot Fuji can do about the physical part of the issue. The only thing they can do is through firmware produce this "turn off/on" message. They do by the way the same for similar issues with card read/write errors (another issue that affects Fuji cameras more than some other brands - which is why Fuji is more critical to using the 'right' SD cards).<<
The issue of the camera/lens contacts is often triggered by a small 'accident' like dropping the camera or bumping it. Heavier lenses (by nature) are more often affected. What amazed me was that it seems not solved in the GFX mount. Obviously the heavier cameras and lenses put more strain to the mount, but with a new engineered mount you wouldn't expect this.
PS. by the way, the "f0" error that I sometimes get on my X-H1 is induced when I disconnect the camera from my iPhone (bluetooth). I hope this was solved in the recent 2.10 firmware under "small bug fixes".
