Hi, I think it turned out the answer was right under my nose the whole time:
"- Layer has an in-memory join or relate (Feature Service)"
It turned out that you couldn't use a layer with a join active on it to author a Feature Service (it's OK for a Map Service though).
I had a specific need to publish a Feature Service, and we ended up with my colleague the database expert creating an SDE 'spatial view' instead, using command-line tools (we use Oracle for our database). The spatial view brings together the geography, plus variou fields from various tables but just to look at it you wouldn't know it's any different from any other feature class.
Esri tech support helped us with getting the spatial view constructed when we ran into a problem.
Hope this helps, good luck!