If you change the host name of your server or, as in my case, install as non-root user under Linux and therefore need to use non-standard ports like "8080" and "4430" in the URL for installing and then later switch that back to the standard port "80" and "443", you might see the problem that your Rich Content Widget in Connections 5.5 Communities no longer works.
The reason for that could be, that in the database there is a reference to the wrong hostname / port number. Check the database table HOMEPAGE.OH2P_CLIENTCFG for an entry "conn-rte". If you find one and it has the reference to the wrong hostname or port, just delete the entry and restart your Connections servers. This is described in that Knowledge Base article.
The entry should be recreated automatically with the correct values and the Rich Content widget should work again.
By the way: There might be another reason why the Rich Content Widget in Connections 5.5 is no longer working. If you have configured Connections that way that all applications need authentication, then make sure that the READER role for the Rich Text Editor WAS application is set to "Everyone" and not to "All authenticated users" (which is then the case for the other applications). Otherwise the widget will not work.
Update: As there were questions what exactly the "Rich Content App" is: It is a new application which you can add in a Community to display rich text content: