Unable to connect through jdbc, sqlcmd connects

Ubuntu 18.04
Sonarqube

I’m trying to connect my sonarqube container to the ms sql server with a instance. With sqlcmd the connection succeeds but sonarqube keeps complaining about the server. Where do I go wrong?

working sqlcmd command:
sqlcmd -S s120.ccs.nl\\sqlprod,1440 -U <username> -p -Q "SELECT @@VERSION"

sonarqube config in docker-compose.yml:

       - SONAR_JDBC_USERNAME=<username>
       - SONAR_JDBC_PASSWORD=<password>
       - SONAR_JDBC_URL=jdbc:sqlserver://s120.ccs.nl\\sqlprod:1440;databaseName=sq_rcip

and are ommitted for obvious reasons.

Part of the output docker-compose up:

sonarqube    | 2020.03.19 14:07:50 INFO  web[][o.sonar.db.Database] Create JDBC data source for jdbc:sqlserver://s120.ccs.nl\\sqlprod:1440;databaseName=sq_rcip
sonarqube    | 2020.03.19 14:08:04 ERROR web[][o.s.s.p.PlatformImpl] Web server startup failed
sonarqube    | java.lang.IllegalStateException: Fail to connect to database
sonarqube    |  at org.sonar.db.DefaultDatabase.start(DefaultDatabase.java:87)
sonarqube    |  at org.sonar.core.platform.StartableCloseableSafeLifecyleStrategy.start(StartableCloseableSafeLifecyleStrategy.java:40)
sonarqube    |  at org.picocontainer.injectors.AbstractInjectionFactory$LifecycleAdapter.start(AbstractInjectionFactory.java:84)
sonarqube    |  at org.picocontainer.behaviors.AbstractBehavior.start(AbstractBehavior.java:169)
sonarqube    |  at org.picocontainer.behaviors.Stored$RealComponentLifecycle.start(Stored.java:132)
sonarqube    |  at org.picocontainer.behaviors.Stored.start(Stored.java:110)
sonarqube    |  at org.picocontainer.DefaultPicoContainer.potentiallyStartAdapter(DefaultPicoContainer.java:1016)
sonarqube    |  at org.picocontainer.DefaultPicoContainer.startAdapters(DefaultPicoContainer.java:1009)
sonarqube    |  at org.picocontainer.DefaultPicoContainer.start(DefaultPicoContainer.java:767)
sonarqube    |  at org.sonar.core.platform.ComponentContainer.startComponents(ComponentContainer.java:135)
sonarqube    |  at org.sonar.server.platform.platformlevel.PlatformLevel.start(PlatformLevel.java:90)
sonarqube    |  at org.sonar.server.platform.platformlevel.PlatformLevel1.start(PlatformLevel1.java:164)
sonarqube    |  at org.sonar.server.platform.PlatformImpl.start(PlatformImpl.java:213)
sonarqube    |  at org.sonar.server.platform.PlatformImpl.startLevel1Container(PlatformImpl.java:172)
sonarqube    |  at org.sonar.server.platform.PlatformImpl.init(PlatformImpl.java:86)
sonarqube    |  at org.sonar.server.platform.web.PlatformServletContextListener.contextInitialized(PlatformServletContextListener.java:43)
sonarqube    |  at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4770)
sonarqube    |  at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5236)
sonarqube    |  at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
sonarqube    |  at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1423)
sonarqube    |  at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1413)
sonarqube    |  at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
sonarqube    |  at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
sonarqube    |  at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
sonarqube    |  at java.base/java.lang.Thread.run(Unknown Source)
sonarqube    | Caused by: java.lang.IllegalStateException: Can not connect to database. Please check connectivity and settings (see the properties prefixed by 'sonar.jdbc.').
sonarqube    |  at org.sonar.db.DefaultDatabase.checkConnection(DefaultDatabase.java:119)
sonarqube    |  at org.sonar.db.DefaultDatabase.start(DefaultDatabase.java:84)
sonarqube    |  ... 24 common frames omitted
sonarqube    | Caused by: java.sql.SQLException: Cannot create PoolableConnectionFactory (The TCP/IP connection to the host s120.ccs.nl, port 1440 has failed. Error: "s120.ccs.nl. Verify the connection properties. Make sure that an instance of SQL Server is running on the host and accepting TCP/IP connections at the port. Make sure that TCP connections to the port are not blocked by a firewall.".)
sonarqube    |  at org.apache.commons.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:669)
sonarqube    |  at org.apache.commons.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:544)
sonarqube    |  at org.apache.commons.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:753)
sonarqube    |  at org.sonar.db.profiling.NullConnectionInterceptor.getConnection(NullConnectionInterceptor.java:31)
sonarqube    |  at org.sonar.db.profiling.ProfiledDataSource.getConnection(ProfiledDataSource.java:317)
sonarqube    |  at org.sonar.db.DefaultDatabase.checkConnection(DefaultDatabase.java:116)
sonarqube    |  ... 25 common frames omitted
sonarqube    | Caused by: com.microsoft.sqlserver.jdbc.SQLServerException: The TCP/IP connection to the host s120.ccs.nl, port 1440 has failed. Error: "s120.ccs.nl. Verify the connection properties. Make sure that an instance of SQL Server is running on the host and accepting TCP/IP connections at the port. Make sure that TCP connections to the port are not blocked by a firewall.".
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerException.makeFromDriverError(SQLServerException.java:234)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerException.ConvertConnectExceptionToSQLServerException(SQLServerException.java:285)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SocketFinder.findSocket(IOBuffer.java:2431)
sonarqube    |  at com.microsoft.sqlserver.jdbc.TDSChannel.open(IOBuffer.java:656)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectHelper(SQLServerConnection.java:2440)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerConnection.login(SQLServerConnection.java:2103)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectInternal(SQLServerConnection.java:1950)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerConnection.connect(SQLServerConnection.java:1162)
sonarqube    |  at com.microsoft.sqlserver.jdbc.SQLServerDriver.connect(SQLServerDriver.java:735)
sonarqube    |  at org.apache.commons.dbcp2.DriverConnectionFactory.createConnection(DriverConnectionFactory.java:55)
sonarqube    |  at org.apache.commons.dbcp2.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:355)
sonarqube    |  at org.apache.commons.dbcp2.BasicDataSource.validateConnectionFactory(BasicDataSource.java:115)
sonarqube    |  at org.apache.commons.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:665)
sonarqube    |  ... 30 common frames omitted