I think I have found the problem. We include our images in the resource file to be included in the build. I had one that apparently was not compatible. I replaced it, and was able to get the build to create an .exe.
For the record:
1) 12.6 build 4058
2) Windows 7 Professional SP1
3) Migrating from 12.5
4) Yes, optimized everything
5) No warnings/errors on full or incremental build
6) 32bit P-Code
