After adding the following remote repository via UI, this error appears in UI and archive.log each time the repository page is loaded:
2017-01-02 16:57:31,927 [http-bio-8000-exec-8] ERROR org.apache.cxf.jaxrs.utils.JAXRSUtils  - No message body writer has been found for class org.apache.archiva.rest.services.ArchivaRestError, ContentType: text/plain
Note: The repository does obviously provide no index (results in 404).
However, fetching artifacts (e.g. http://download.oracle.com/maven/com/sleepycat/je/5.0.104/je-5.0.104.pom) is working ok.
Besides the above error, requests to Archiva are proxies without problems and artifacts are retrieved without error from that repository.