No VPN, no proxy
You might need to submit a ticket to Zwift Support.
resetting internet options does not work, I will try to ask Zwift support
thank you all for your advice
Problem now solved for 1 month - I had to buy BigringVR subscription when Zwift does not work 
In the Documents\Zwift\Logs folder there should be some log files. The latest one of those might contain some clues.
e.g. unable to load texture file…
The files should normally be in something like C:\Program Files (x86)\Zwift\assets
Best bet is probably to start a support ticket as suggested (https://support.zwift.com/) . Include the log files if you can upload attachments.