The article applies to Lync Server 2010 and Lync Web App.

Scenario

A Lync meeting is created and a link is opened on a machine with no Lync / Skype for Business client installed, so Lync Web App is used.

If the Join as a guest option is selected, the app gets stuck on Joining the meeting status and then the There Was An Error While Joining The Conference. Please contact the meeting organizer error shows up.

Join as a guest

If the Join using your corporate credentials option is selected, the the app gets stuck on Joining the meeting status and then the An Error Occurred While Trying To Join The Conference error shows up.

Join using corporate credentials

Explanation

Lync Server 2010 mainstream support ended on April 12, 2016. As a result, Lync Web App is no longer supported in the vast majority of modern browser/OS.

The issue doesn't occur in Skype for Business Web App, so upgrade to a later Exchange version will resolve the problem. Read the Knowledge Base article on In-Place Exchange 2007/2010/2013 To Exchange 2013/2016 Migration Procedure for more information.