Without proper system info or details like your saving options, file paths and file names, slice names etc. nobody can say anything. You haven't even told us what version of PS, so pardon me, but this is totally useless.
Apologies, first time I have used this forum. No luck trying to reach Adobe for tech support.
I am using CS6 on a 64x Windows PC, Windows 7 Enterprise. Issue is with Photoshop.
I am attempting to perform a very basic "Save for web" function. Action - "File", "Save for Web" then get the d'box shown above.
Was working fine yesterday, not today, no changes to system, no logical reason for a change.
I tried a re-install, made no difference, still occurring.
That's all I can say...