Hi,
You did not say which browser or saving method you are using. However, this sounds similar to something that happened to me sometime last year.
In my case, saving with Timimi on Firefox suddenly stopped working in my work environment only. This was not due to any change in TW or in Firefox, because I could use Firefox Portable to set up identical Firefox configurations on my home and work machines and only work failed to save.
It seems that the problem resulted from some change that our IT people made to the networked system at work that was beyond my control. I could never find out exactly what might have changed that would have this effect; most likely one of their frequent "security" upgrades.
I was able to work around the problem by using TiddlyDesktop for loading and saving TWs at work; perhaps that will work for you.
--Jon