Official Luthiers Forum!

The UTB is 5/8h x 1/2 w. Top is .093ish. Thinner than I usually go but not unheard of.

If it is the soundhole area that is folding up as it does on some of the old gut strung guitars that are strung with steel strings, you may be able to reinforce this area by glueing in fairly massive bracing around the soundhole, between the upper arms of the X and the UTB. Fit them in tight against these braces and don't skimp on the glue. You should be able to add them through the soundhole without having to take the guitar apart. Some people notch the UTB and carry these braces into the neck block, but at this point that may not be practicle.
Another approch might be that of the "Tilton patent improvement" - running a dowel from the neck block to the end block, not a very elegant solution.

Hmmm... If I could use turnbuckles instead if a dowel to get it back into shape and then butt the UTB against a side brace ....

Once you get things back into position you could run braces from the top of the neck block to the back/sides at the waist similar to what Rick Turner, Sam Littlepage and others have done. The Mimf library has some information under "flying buttress bracing" and "frame guitar". Again this work could probably be done through the soundhole.
Unless the guitar is folding along it's centerline I would be surprized if the failure is due to the end attachment of the UTB.