The only thing I can think of that might be causing this would be a corrupted file somewhere. All the files related to the Cleft Age should be in the dat folder in your wineskin. Because this is played standalone (not with others on the shard), I'm thinking it *should* be ok to overwrite them with ones from a working installation of UruCC. Before taking this step, you may want to make a backup copy of your DI wineskin, just to be safe.
At least, this is one of the troubleshooting steps I would try (or the more tedious business of comparing each Cleft*.prp file size between a working UruCC install and the DI install). It's difficult to know which particular prp file is associated with the hidden cave under the cleft, though there are many that you can rule out by their descriptive filenames. The ones that look the likeliest to me are:
The only other place I see likely files for the Cleft Age is in the SDL folder (Cleft.sdl), so you might want to replace that one too.
Sorry I can't be more helpful.
Please post here if anything helps, or if you are able to rule out some of the possibilities.