Oh dear. Thats a drastic change! Unfortunately what I'm going to tell you isn't going to help, as you'll either have to take it out to save it or... well, yeah.
Ficus indoor plants do well with
indirect light - meaning smaller ficus plants in your type of enclosure would do best because they'd have some distance between the light and the branches. I originally had my ficus plant directly under my 3 lamps and it started to shed leaves just like this, but then I moved them to the other side of the cage (only a couple inches) and it started to thrive again. Unfortunately it looks like the plant might be a bit big to do this, but you can try to move the lights to one side of the enclosure, and try to create as much distance that way.
Other problems would be drafts if the enclosure is located next to a window - they don't tolerate this well. Just like your cham, it needs a high humidity - so if there is a lower than recommended humidity for your cham hike it up!
Also “sap” dripping from a ficus houseplant, which is actually honeydew from an invading pest, is a sign of an infestation. Treating the plant with neem oil is a good way to handle any of these pest issues. But I'm still putting money on the fact that its the light so close to it - but just in case, thats a note there in case you see "sap"!
What you could also try for the light, since its a beautiful plant with its leaves and fits that enclosure very nicely, is finding a thick but porous (non-flammable) material (so screen or something of the like) and secure it above the plant, particularly where that UVB lamp is - not of course directly under it. But like have a rectangular piece hanging down from the top so it creates a 90degree angle (a quick graphic here is going to help... but bear with me its going to be quick and crap!)
Alright, so the yellow, is obviously your UVB lamp, and the red is the piece of screen you'd have hangling down to block some of the light from one part of the plant. It wont save all of the plant, but it'll hopefully encourage some growth on the other side of the cage. And your cham will be able to choose his spots. He will recognize it as a sunny vs. shady area and regulate himself accordingly, so you dont have to worry about that!
Otherwise, thats as much info as I can give you! Good luck! It'll have a quicker recovery if you take it out for a couple weeks and put it beside the enclosure, then put it back in once you've gotten that piece figured out, but then it would be fairly empty for your cham... so its a toss up!
This is an old setup of mine, but I have two small ficus plants in it - one top left, and a bigger thriving one bottom right that is hidden from most of the light by my dracaena and my purple wandering jew. I've since changed it so that both ficus are near the bottom, but this set up still wasn't bad because while the left ficus wasn't thriving, it wasn't dying either at that distance.
That said, if it ever gets to a point that you just want to say screw it with the ficus, I highly recommend a dracaena tree/plant - they come in various different variations and are one of the hardier tropical plants I know of. (When you got to buy it it'll always say Dracaena ______ - and this is what I mean that it comes in so many different variations, and most of them wont look the same, but any Dracaena is non-toxic to chams and hardy as hell) You can't over water it because it will just drain out, it likes moderate to high sunlight (though if its getting too much light it will get brown tips - but wont drop the leaves completely). Only problem you'd have with this one is that it grows FAST - so you'd have to look at pruning it back every 4 months or so so it doesn't overcrowd your cage. But other than that... yeah!
Good luck! Hope that helped!