I had a misspelling in the pom.xml. After rewritting the corresponding block, it worked :)
Before:
<plugins>
<plugin>
...
<configuration>
<id>ec2-node</id> <-- Wrong
...
After:
<plugins>
<plugin>
...
<configuration>
<serverId>ec2-node</serverId> <-- Right
...
The addressed auth information, defined in the settings.xml, worked instantly.