I started experiencing this same problem last year - it seems to crop up intermittently - and now again with my new Photoshop CS installation. Fortunately I solved the problem for myself back when it first happened, and posted about it, and so only logged on here to refresh my memory. Since no other suggestions worked for me, I thought I would repost the thing that finally did work, in case it might help someone else. I just tried it in PS CS, and it still works!

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scamper - 01:01am Jun 13, 2003 Pacific (#2 of 2)

Same problem here, only I think I may have found the answer - at least for me.
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After reading the Forums here I tried trashing the "Adobe Save For Web 3.0 Prefs" file, but it did no good. I even saved a dummy file with that name and locked it, but PS doesn't seem to care either way. I think that is a red herring, honestly.

No, the *only* thing that I've found that has any positive effect is to trash my custom Actions.atn file! The correlation is as direct as this: If I trash all PS-associated files, start Photoshop, open a file, and Save for Web, everything runs flawlessly. But when I try the exact same thing after loading (Replace) my custom Actions, suddenly Save for Web is broken (both the Action and the menu item).

I've now rebuilt my custom Actions again from scratch, and saved them into a new .atn file. So far everything is working.

I did add a new action (the first time I've done this in quite a long while, and unrelated to Save for Web) a week or so ago... maybe it just got corrupted. Or maybe Adobe Online changed something minor with the program so that my Actions somehow came into conflict.