504 error on uploading artifacts to staging

For some weeks now, I have been trying to deploy artifacts via mvn release goal. It was working fine for a while then suddenly stopped.

Outcome is same. Mvn tries to deploy first artifact (975kb):

[INFO] Uploading: http://oss.sonatype.org/service/local/staging/deploy/maven2//org/jclarion/clarion-runtime/1.76/clarion-runtime-1.76.jar

After some network activity during upload, then network is silent for about 2 minutes. Then I get this error:

[INFO] [INFO] Error deploying artifact: Failed to transfer file: http://oss.sonatype.org/service/local/staging/deploy/maven2/org/jclarion/clarion-runtime/1.76/clarion-runtime-1.76.jar. Return code is: 504