Thanks, this is perhaps progress…
Could you elaborate a bit on how you identified the problematic channels? I’ve got tons of offline processed parts, don’t know how and where to start troubleshooting. I’m considering importing tracks to a blank project, one by one… 
EDIT: Workaround. I successfully imported all tracks to an empty project and did it all with a single import. I don’t know if it’s some individual tracks causing the issue.