I am facing a similar issue after upgrading to 15.6, behavior is as follows:
Clicking on edit if WYSIWYG is set as default edit mode, with RealtimeCKEditor being set as default, results in the following console error:
Failed to load resource: the server responded with a status of 404 () xwiki-realtime-wysiwyg-inline.js:1
This confuses me a bit since afaik inline editing is not supported yet?
If i now manually select WYSIWYG from the editing dropdown, a Realtime Session opens and connects in standalone mode. This session works as intended with showing other connected users, allowing collaborative editing etc. However, any changes made will be discarded during the save and another error is thrown, with regards to https://xwikiurl/path/to/page/xwiki-realtime-wysiwyg-inline.js net::ERR_ABORTED 404.
I have updated the official way from 15.4 to 15.5 and now to 15.6, installed via tomcat9 package from the deb distribution, no changes where made to any config or files, all extensions have been updated to the current version.