Recently, after updating my Mac from Snow Leopard to Yosemite, I was chagrined to find that Adobe Photoshop's Save for Web -- a feature I rely on heavily -- would not save a file. Every time I tried it, I got the message, "Could not complete this operation. An unknown operating system error has occurred."
Scouting around the Web, I found that the common solution was to delete the two Adobe Photoshop Save for Web preference files. Sometimes that worked for people, and sometimes it didn't, and it certainly didn't for me. But digging into my system, I found why: There are three possible locations for Adobe Photoshop Save for Web preference files, not two! And once I knew that, I fixed it easily.
So, for a guaranteed fix, close Photoshop and delete all the following files and folders, and any others that look like them -- for different versions or even different Adobe apps.
Users/[Username]/Library/Preferences/Adobe Save For Web 13.0 Prefs
Users/[Username]/Library/Preferences/Adobe/Plugins/Adobe Save for Web CC
Users/[Username]/Library/Preferences/Adobe Photoshop CC Settings/Save for Web Prefs.psp