OK. I apologise for ever even considering it was one of your mods. The damned thing is still happening every time I try to marry Mortimer and Dina. Letting the second timer run down results in an automatically called nanny (even though Cassandra is still living at home) and they go on holiday. Any advise on how to stop the stupid thing would be gratefully received. I'll come back here and post whether or not I get errors when I try to marry other Sims.
this happened in a vanilla game for me for Cassandra and Don - I let them have a crappy wedding party while they chilled for a few hours, and at the end of it the shuttle arrived. I suspect it had to do with the fact that the wedding ended late at night, and the second party took them to a decent time the next morning - or that's how I explained it away in the story in my head anyway, that they had to wait for morning for the shuttle.