Intellij: Not a valid project ID:

Using IntelliJ Idea Ultimate 2020.2 none of the above worked for me.

I had to

• Delete the .idea folder
• Restart IntelliJ
• File -> Project Structure
• Create a new play module at the same location

I could solve the problem:

• Close Project in Intellij.
• Delete .idea folder of the Project.
• Open the Project again.

In my case, the reimporting of sbt project fixed the issue. This can be done by clicking on the "sbt" tab on the right pane, right-click on the project and selecting "Reimport sbt Project".