NullPointerException on HTML View since 3.21.2

After upgrading our OSS Sonatype Nexus to 3.21.2 (previously on 3.18.2), the HTML View of our maven release repository is returning a HTTP 500 error.

nexus.log shows a NullPointerException Java stack trace :

UNKNOWN org.sonatype.nexus.siesta.internal.UnexpectedExceptionMapper - (ID aa5f3120-0e16-4156-873e-5671464afb81) Response: [500] 'ERROR: (ID aa5f3120-0e16-4156-873e-5671464afb81) java.lang.NullPointerException'; mapped from: java.lang.NullPointerException

Full log in attachment.

This happens at the root path of the repository. Browsing other subdirectories seems to work fine.
I suspect that an empty "folder" located at the root of the repository and named D:\archives\SVN\mimosa\mimosa_c++\v6 might be responsible of the issue.

I have not been able to delete this folder using the UI and the delete folder button.