The problem:
Valid license keys are being incorrectly identified as invalid during the confluence setup process.

When running confluence in standalone mode, license key X works fine.
When running confluence from within IDEA, that same licence key is being marked as invalid.

Workaround:
Extracting the com/atlassian/confluence/page/Page.key file from atlassian-extras and placing it at the head of the classpath. This implies that another (invalid) Page.key file is being picked from somewhere else.