Thanks for your tip! Will do that in the future. I have worked around it by copying all the artboards to the previous Sketch version. Messed up the content, but yeah...

I Have contacted them and they said that they'd fix it indeed. Meanwhile I have copied everything from Sketchfile 44 to 43.

Found that out the hard way now... Isn't there a way to convert the document?

Got a big document with many artboards that is very slow now. The thing is I can't go back to the previous sketch because it won't open in 43. This is a real bummer.