I sent a PDF for shared commenting using Adobe acrobat Pro DC. Reviewers are using Acrobat XI. None of the reviewers are able to post comments and get errors saying "Acrobat did not get a response from the server. Try again later or contact your system a

Security says 'commenting allowed'.

Reviewers are using compatible version of Acrobat.

Reviewers have access to server.

I am seeing this error in the tracker when coworker sent a shared review PDF to me:

Using Windows 7 enterprise.