Skype for Business clients can get into a scheduled meeting, but Web App can't

I have a Standard Skype for Business pool, where the Web App doesn't work

When users access the meeting url, and don't have a Skype for Business client installed, the Web App report this error:

Sorry, that didn't work.
There was an error in contacting the service. Please contact your administrator.

If the SfB client is installed, it work as expected

This happens for both internal and remote users

Any ideas to troubleshoot this?