I have encountered the same issue.And as I checked from google, the same issue also happened in 2018:

  
      github.com/conda/conda
  
  
    
  
	  
  

  
    
      CondaHTTPError: HTTP 503 SERVICE UNAVAILABLE: BACK-END SERVER IS AT CAPACITY for url <https://conda.anaconda.org/conda-forge/linux-64/repodata.json> 15
    

    
      
        opened 
        
          
        
        Sep 24, 2018
      
      


      
        
          
          jakirkham
        
      
    
  



  Current Behavior
$ docker run --rm -it condaforge/linux-anvil
$ conda create -n cmake cmake
Solving environment: failed
CondaHTTPError: HTTP 503 SERVICE UNAVAILABLE: BACK-END SERVER IS...





  
  
    
    
  
  


Waiting for Anaconda.org 3 recovery.