I found a workaround, which is to create a new project in 10.5 and then import all the tracks from the old project. The project will play after that - but unfortunately, it seems the import function sometimes does not bring the automation status with it (i.e. the green R is not set on some tracks). I’ve reported that as an issue. At the moment the only workaround to that seems to be to manually cross check the automation statuses between the old and the new projects. But at least it’s possible to work on the project in 10.5.
On the other thread there is a workaround which only works on old projects that have not yet been saved in 10.5, which may be better in that case as it doesn’t involve an import.
R