Thank you to Sunlight who discovered how to connect to ETPRO server if you have problems to connect there!
In the client:
cl_wwwDownload 1 is optional but you will download much faster.
I also discovered the reason why I couldn't connect from Linux, if that can help.
In my etpro/etconfig.cfg I had cl_wwwDownload 0
Therefore even if I connected with +set cl_wwwDownload 1, I started from etmain and once etpro was loaded it loaded its own etconfig, and cl_wwwDownload went to 0 thus changing my userinfo and aborting the www download from the server. Since in the server there was a sv_wwwFallbackURL set, the whole thing failed.
now sv_wwwFallbackURL is empty and if you fail the download from www you will download slowly from the ET server itself.
Make sure to have cl_allowDownload 1 cl_wwwDownload 1 both in etmain's etconfig.cfg and etpro's etconfig.cfg, or even better, connect with +set fs_game etpro or load etpro first, then set cl_allowDownload 1 cl_wwwDownload 1 and connect.
Anyway, now you will be able to connect regardless, but that would allow you t odownload from www which is much faster.