title: Can't install any package that includes tqdm
body: <p>The last couple of hours I've been updating my python and anaconda packages, but I have hit a wall regarding this tqdm package. The first thing I tried was installing nbformat with: 
  <code>conda install nb format</code></p>
  
  <p>and got back:</p>
  
  <p>The following NEW packages will be INSTALLED:</p>
  
  <pre><code>conda-package-handling: 1.6.1-py36h62dcd97_0
  tqdm:                   4.46.0-py_0
  </code></pre>
  
  <p>The following packages will be UPDATED:</p>
  
  <pre><code>conda:                  4.3.30-py36h7e176b0_0 --&gt; 4.8.3-py36_0
  conda-env:              2.6.0-h36134e3_1      --&gt; 2.6.0-1
  menuinst:               1.4.10-py36h42196fb_0 --&gt; 1.4.16-py36he774522_0
  nbformat:               4.4.0-py36h3a5bc1b_0  --&gt; 5.0.6-py_0
  pycosat:                0.6.2-py36hf17546d_1  --&gt; 0.6.3-py36he774522_0
  vc:                     14-h2379b0c_2         --&gt; 14.1-h0510ff6_4
  vs2015_runtime:         14.0.25123-hd4c4e62_2 --&gt; 14.16.27012-hf0eaf9b_2
  </code></pre>
  
  <p>Proceed ([y]/n)? y</p>
  
  <p><code>ERROR conda.core.link:_execute_actions(337): An error occurred while installing package 'defaults::tqdm-4.46.0-py_0'.
  CondaError: Cannot link a source that does not exist. C:\Users\sergi\Anaconda3\Scripts\conda.exe
  Running conda clean --packages may resolve your problem.
  Attempting to roll back.</code></p>
  
  <p><code>CondaError: Cannot link a source that does not exist. C:\Users\sergi\Anaconda3\Scripts\conda.exe
  Running</code>conda clean --packages<code>may resolve your problem.</code></p>
  
  <p>I tried updating conda, but I get the same thing: </p>
  
  <pre><code>ERROR conda.core.link:_execute_actions(337): An error occurred while installing package 'defaults::tqdm-4.46.0-py_0'.
  </code></pre>
  
  <p>CondaError: Cannot link a source that does not exist. C:\Users\sergi\Anaconda3\Scripts\conda.exe
  Running <code>conda clean --packages</code> may resolve your problem.
  Attempting to roll back.</p>
  
  <p>Finally I tried installing this tqdm package with <code>conda install -c conda-forge tqdm</code>, got the same error again. </p>
  
  <p>I have also tried with <code>conda config --add pinned_packages defaults::conda</code>, but nothing seems to work. </p>
  
  <p>If anyone has any idea of what's going on I would greatly appreciate some help.</p>
  
  <p>Thanks!</p>