Slashes appended to remote repo url

Archiva is used with remote repositories. I added two repo urls

• http://nexus.thenewmotion.com/content/groups/public/
• http://repo.maven.apache.org/maven2/
  Nexus repo is fine, maven central is not.
  Logs only show this:

2017-05-02 12:51:31,265 [qtp2059174882-28] 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

So I enabled debug logs (in log4j2.xml), and now I see this

2017-05-02 13:40:38,286 [Thread-49] DEBUG org.apache.archiva.redback.components.taskqueue.DefaultTaskQueue [] - pool
2017-05-02 13:40:38,300 [qtp725397266-25] DEBUG org.apache.archiva.proxy.common.DebugTransferListener [] - wagon debug http://nexus.thenewmotion.com/content/groups/public/// - Status code: 200
2017-05-02 13:40:38,325 [Thread-42] DEBUG org.apache.archiva.redback.components.taskqueue.DefaultTaskQueue [] - pool
2017-05-02 13:40:38,386 [Thread-49] DEBUG org.apache.archiva.redback.components.taskqueue.DefaultTaskQueue [] - pool
2017-05-02 13:40:38,425 [Thread-42] DEBUG org.apache.archiva.redback.components.taskqueue.DefaultTaskQueue [] - pool
2017-05-02 13:40:38,445 [qtp725397266-25] DEBUG org.apache.archiva.redback.rest.services.interceptors.ThreadLocalUserCleaner [] - handleMessage
2017-05-02 13:40:38,459 [qtp725397266-23] DEBUG org.apache.archiva.proxy.common.DebugTransferListener [] - wagon debug https://repo.maven.apache.org/maven2/// - Status code: 404
2017-05-02 13:40:38,460 [qtp725397266-23] DEBUG org.apache.archiva.redback.rest.services.interceptors.ThreadLocalUserCleaner [] - handleMessage
2017-05-02 13:40:38,460 [qtp725397266-23] 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

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Seems that Archiva appends two slashes at the end of my urls.
Somehow nexus works, allows those extra slashes.
Maven Central doesn't.

I get the same error if I do not add myself one trailing slash.
I still get two slashes at the end, and Maven Central query results in 404.