I changed a layout to not consolidate multibar rest and now I get this…
What to do ?
Click on the first note after the multi-rest. It’s probably set to the “Starts Voice” property. Un-toggle that.
Thank’s, Dan! - that was close enough to fix it 
It was the note before the multirest that was set to “End Voice”.
I don’t get the logic behind this…
If you don’t have any active voices on a stave, Dorico doesn’t know how many beats each bar is supposed to last. The bigger question is how you ended up in this situation: the likely answers are that either you used Remove Rests and then moved material around, or it’s a musicxml import that’s afflicted with this common glitch.
It was a musicxml import from Sibelius 7 via the dolet plugin.
MusicXML represents the appearance of the notation, not its semantics. The MusicXML file will most likely contain explcit bar rests in every empty bar.
In Edit / Preferences / MusicXML Import, there are three options to choose what Dorico does with imported rests, varying from copying the original notation exactly, to ignoring them and using implicit rests in the Dorico score.