Hi Tobias,
Sprintr Project not found probably means https://sprintr.home.mendix.com/index.html in here the project is deleted or removed and now you are not able to find it.
You can check if the source folder contains .mpr file and open it with mendix version selector.
In future you should always create a .mpk or .mda file so that you just need to import it and use it.