A great time saving post on MS SharePoint Designer Team blog:
How to fix: Recurring Meeting Workspace error: ‘g_InstanceID’ is undefined
I ran into this problem after applying a custom master page to a meeting workspace, which was linked to a recurring event. Date selection on the left navigation stopped working if a custom master page was selected, but it worked fine when site master page was set to a standard SharePoint one.
Adding these two lines to a custom master page, mentioned in the blog post above, resolve the issue:
<%@ Register Tagprefix="Meetings" Namespace="Microsoft.SharePoint.Meetings" Assembly="Microsoft.SharePoint, Version=18.104.22.168, Culture=neutral, PublicKeyToken=71e9bce111e9429c"%>