K. That didn't work. Here's a new description that may help...
I opened the VDL Template, set the instrumentation of all percussion (Sn Tn Bs, Xylo, 3 Mar, 3 Vib, Timp, 2 Rack) and wrote with no problems. Figuring then that since this file was "safe", if I started with this file and added the wind instruments from the natural Sibelius 7 Sounds library, I could resume scoring for the full marching band. So I deleted all the music from those percussion staves, as well as deleted the staves I no longer needed, and I added the Wind instrumentation (Picc, Flute, 2 Clar, Alto, Tenor, 2 Tpt, Horn, Trombone, Euph and Tuba). Saved. Changed the key to "Atonal/No Key" for the piece I wanted to work on. Tried to save, and got the attached error message. Quit without saving. Re-opened the same file, and before trying to change keys or anything again, I tried to save (having done nothing) and got the same error message again.
This is an error message I have received several times in the last week since these errors began occurring. Again, all of this happened based on the template file, with no changes to fonts or any other settings besides what is described here.
Also: I have uninstalled and reinstalled, through this process: Sibelius 7, Sibelius 7 Sounds, Sibelius 6 (which I seem to be unable to register again?), Sibelius 6 Essentials, Kontakt 5, VDL 2.5.5, and upgraded to Mountain Lion.