Hi,
I learned that this is because of migration from CRM 2011 to 365 where Workplace and Ressource Center do not exist anymore. I also have double-checked with a brand-new solution inside 365. And yes, there is no Workplace or Ressourcecenter anymore. I also
have replaced the workplace_24x24.gif with something else in C:\Program Files\Microsoft Dynamics CRM\CRMWeb\_imgs\Navbar\ActionImgs\ and it works. Just that I have not enough knowledge about icons and levels etc... and in order to make this work properly,
resize when required, etc. you need more than just a nice icon.
But if one knows how to deal with it, he just needs probably the correct icon size etc. and can replace it, or eventually even change reference with XrmToolBox SiteMapEditor