Quote -   I also tried bevel on those edges (as I've read here in the forum that the solution is extra edges in corners) but things get even worse! 

     Gets worse?  Hmmm, beveling will commonly create n-gons;  did you divide those n-gons up into quads & tris?  N-gons bordering a concave edge will render as spanning the concavity.  For instance, a single poly shaped like a star of David (six point star) will render as a hexagon, and the UVs will be scrambled.

     In your initial post, the arch with the smoothing on seems to show this behavior, right where your blue arrows are pointing.  Another possibility is that the side walls are separate from the vault ceiling, in which case the vault ceiling will be rounded but the outline of the sidewall will not.  You'll also not get contiguous UVs.

     When the arch object is selected in the modeling program, there should be some list of how many quads, tris, and n-gons the selected object has.  Use that feature to check for n-gons.  Any there?

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5