This is an old problem that did not happen many times.

export the project in v12, import it in v12, then export this again and import in v15.

If error is there press ok and see if you still can see the project.

the problem is the error is when i export first time, but i will do it!

when i try the first export at v12 after 70 % i get the message and nothing else from there!

Then there is only one thing to do, I don't know if you like it, but it might help.

there is possibly on 1 page an error.

Make a copy of the project.

remove page by page from the copy project or if you have many pages 5 peaces by 5 or something like that, do this until the export works.

This way you should be able to find out on what page the error is.

after that you need to remove it from the original, or better yet make another copy and remove it from there.

Import and the remake the lost page manually.