Hi, I'm having the same problem with Mathematica v10.0.1.
I asked Wolfram Customer Service for help, and I got this answer (attached from here on)
Error 1000 is almost always caused by a known problem that will be fixed shortly. There is a workaround that I'll copy/paste below:
Some users may be seeing an error message that says the following when they attempt to save something to the Cloud in either Wolfram Desktop or Mathematica on Windows:
"File failed to save: error 1000"
This was reported in 10.0.0. I have received a workaround from a developer (below), and the problem will be fixed in a minor release in the not-far future.
This should work on Windows:
The problem is essentially caused because \
$TemporaryDirectory and Environment["Temp"] are returning different things. So \$TemporaryDirectory will return something like C:\Users\foobar\AppData\Local\Temp. Environment[TEMP] might return something look like C:\Users\foo~1\AppData\Local\Temp. Technically these are the same path to the OS, but the kernel freaks out because they are not the same string even though the OS will interpret them as the same path. You can fix this by setting the operating systems TEMP and TMP environment variables to something shorter.
One approach would be to tell the user to simply create a different user account name that is less than 8 letters or so. But they might not be willing to create dummy accounts just to use Mathematica properly.
Approach number 2 is to set TEMP and TMP to something shorter. You can either do this for Mathematica specifically, with the drawback that you have to do it each time you launch Mathematica, or for the whole OS, with the drawback that you do it for all programs.
If you want to do it for Mathematica specifically:
1) Go to command prompt
2) Navigate to Mathematica installation directory
3) Evaluate: set tmp=C:\TEMP
4) Evaluate: set temp=C:\TEMP
5) Launch Mathematica from command prompt
The problem should be fixed as the temp directories is C:\TEMP.
If you want to do it globally, you can perform a web search for articles which describe how to do this on various versions of Windows.
Hope it helps, good luck