Urgent: Storyline 3 - file suddenly can't be opened any more!?! - please help

I have been working on an elarning project with storyline 3 in the recent weeks and months. The last time I opened and worked on this project was this morning. I edited, saved and closed the project, everything was perfectly normal.

A couple of hours later I try to open the project file again, with no success. I keep getting an error message saying something like:

"This project cannot be opened because it contains features that are not available in this version of Storyline, namely:
Improved automatic text field adjustment"

Today I downloaded the newest build of storyline 3, but I have not installed it yet. Older chapters of my elearning project (i.e different storyline files) are opened without any problems.

As I have put a lot of work in this and the project os close to its completion (and its deadline) this is really serious for me.
Any help is very appreciated,
Kind regards,

Frank