I have the same problem. But I think it relates to a regression in curl updates that were just pushed (?).
a) whonixcheck on the whonix-gw (sys-whonix) shows everything is fine;
b) whonixcheck on the whonix-ws (anon-whonix) shows the error that the first commentator is noting
c) whonixcheck -v output in the workstation shows:
[INFO] [whonixcheck] anon-whonix | Whonix-Workstation | whonix-ws Template-Based AppVM | Wed Aug 3 17:08:25 UTC 2016
[INFO] [whonixcheck] Input Detection: INPUT_AUTO=true CLI=true GUI=false
stdin connected to terminal. Using cli output. Not using gui output.
Alternatively, if want to run from command line, but still use the graphical user interface for input, you could add to command line: --gui
[INFO] [whonixcheck] Root Check Result: Ok, not running as root.
[INFO] [whonixcheck] Pin torproject.org certificate: disabled.
[INFO] [whonixcheck] Qubes Settings Test Result: Ok. (GATEWAY_IP: 10.137.3.1)
[INFO] [whonixcheck] Qubes Settings Test Result: Ok, qubes_vm_type is AppVM.
[INFO] [whonixcheck] Check Kernel Messages Test Result: Found nothing remarkable, ok.
[INFO] [whonixcheck] Check whonixsetup Result: done, ok.
[INFO] [whonixcheck] Check Package Manager Running Result: None running, ok.
[INFO] [whonixcheck] check network interfaces Result: Ok.
[INFO] [whonixcheck] Tor Check Result: Not running on Whonix-Gateway, ok.
[INFO] [whonixcheck] Tor Config Check Result: Not running on Whonix-Gateway, ok.
[INFO] [whonixcheck] Tor Pid Check Result: Not running on Whonix-Gateway., ok.
[INFO] [whonixcheck] Tor SocksPort Reachability Test Result: Reachable. (curl exit code: 22 | curl status message:  - [HTTP page not retrieved. The requested url was not found or returned another error with the HTTP error code being 400 or above. This return code only appears if -f, --fail is used.])
[ERROR] [whonixcheck] Tor Bootstrap Result:
Tor’s Control Port could not be reached!
Confirm that Whonix-Gateway is running.
Run whonixcheck on Whonix-Gateway and confirm success.
Rerun whonixcheck here in this Whonix-Workstation.
(tor_bootstrap_status: Socket error: Received empty socket content.)
(Tor Circuit: not established)
Everything was fine just before those updates were installed. I was on testers repository though. Changing the repository to stable and re-running apt-get update && apt-get dist-upgrade doesn’t solve the problem.
Neither does creating a new workstation appVM.
Ironically, Tor Browser works just fine - both 6.0.3 & the new hardened 6.5a2.
I’m sure Patrick will have an answer