Cannot download sources - "Sources not found for: <Artifact Name>"

Problem

When clicking the "Download Sources" banner at the top of the Editor page:

sources are not downloaded and the "Sources not found for: <Artifact Name>" error is displayed.

Cause

A possible cause is corrupted repository indices. IDEA expects the sources to be in a certain repository, but they are not present there.

Resolution

• Reindex the corresponding repository (i. e. Maven Local).

or

• Remove the Maven folder found in the System Directory to force IDEA to create all indices from scratch.