After starting up a fresh instance with default configuration, I copied parts of my existing repository to Archiva's default internal repository. Then I used the "Scan Repository Now" button on the repository administration page. Afterwards, all artifacts were visible on the "Browse" page. So far so good.
I then copied some more groups of my existing repo to Archiva's default internal repository, and used "Scan Repository Now" once again. But this time, the "Browse" page did not show the newly added groups. This was quite surprising, as the log output of RepositoryScanner clearly showed that Archiva DID walk over these new artifacts, without errors or warnings. I tried to use the "Update Database Now" button, but still no effect.
I finally touched one of the POM files in the missing groups, i.e. I gave the POM file a new timestamp. After doing "Scan Repository Now" again, the artifact appeared on the Browse page.