It is fine to open the project in advance versions. If you have not committed anything in 8.18.1, then you can delete the project from the project directory and try downloading the project in the version 8.15.1 you have created.
Otherwise open from Mendix Developer Portal via version selector directly.
Hope this helps.
When that happens to me, I just go to “My app projects” in the Mendix Developer Portal with my internet browser, open the app and then click on “Edit in studio pro”. In that way you can use the Mendix Version Selector when opening the application and select the correct one.
That works for me when I have problems with different versions. Hope it works for you too.