I'm here to share some practical wisdom with PTSai users!
I ran into "failed to save canvas" problem recently, and kind of figured it out.
It might not work for everyone, but it's worth to give it a shot!
First of all, if you have crack version such as Paint tool SAI English pack, then you might have additional simple green icon as sai.exe in the original folder. When you cannot save .sai file or any other beside .psd, then try opening that second green icon. Nothing should disappear. Now try saving through save (S) or save as... . If not, keep trying to save until it's done!
Second tip: try saving on different disc C:\ or E:\.