PlanetsCentral

there is no ship #356 in North Star 8

Well that's embarassing. I habitually name my ships incorporating the ship number and I made a typo in the ship name (I called it ship #356 but in fact it was #353). And it would have been the same ship that I tried to BeamUpMultiple with previously. That explains why I used the "wrong" ship number both times. Using the correct ship number now the turn is accepted without warning.

Thanks Stefan, and sorry for the trouble.