We faced the same issue also. When we ran the httpstat -s 1, we saw that it was maxed out on the controller.
Was able to trouble by performing the following:
1. dfm options set snapDeltaMonitorEnabled=no
2. Setting the httpd timeout to 30 and max connections to 1, then back to their default values once the count was down to 0.
By running this appears to have fixed our issue.
Can also test by running the zexplorer on the dfm server to see if the API is running.