If you want to use your own custom master page you can fix this by merging the missing code located from the mwsdefault.master and put it into your custom master page. The Best Practice is to create a feature to automatically set Meeting Workspaces to your modified custom master page file.
To fix this problem in your custom master page you need to:
1. Copy the register Microsoft.SharePoint.Meetings tag prefix from the mwsdefault.master and add it to your custom master page.
<%@ Register Tagprefix="Meetings" Namespace="Microsoft.SharePoint.Meetings" Assembly="Microsoft.SharePoint, Version=220.127.116.11, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
2. Then copy the Meetings:PropertyBag control into your custom master page.
3. Save master page, check in (if necessary), and publish the major version or deploy it in your feature.