BTW; I'm seeing in the log files that just when the form is being opened the following error is produced:
org.codehaus.jackson.JsonGenerationException: Current context not an ARRAY but OBJECT at org.codehaus.jackson.impl.JsonGeneratorBase._reportError(JsonGeneratorBase.java:375) at org.codehaus.jackson.impl.JsonGeneratorBase.writeEndArray(JsonGeneratorBase.java:152) at aV.f(SourceFile:132) at dO.a(SourceFile:215) at dO.a(SourceFile:199) at dO.a(SourceFile:137) at dO.a(SourceFile:84) at kx.a(SourceFile:52) at jx.a(SourceFile:299) at jx.a(SourceFile:230) at jx.processRequest(SourceFile:174) at fI.a(SourceFile:71) at com.mendix.core.MxRuntime.processRequest(SourceFile:938) at com.mendix.m2ee.server.handler.RuntimeHandler.handle(RuntimeHandler.java:42) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:113) at org.eclipse.jetty.server.Server.handle(Server.java:334) at org.eclipse.jetty.server.HttpConnection.handleRequest(HttpConnection.java:559) at org.eclipse.jetty.server.HttpConnection$RequestHandler.content(HttpConnection.java:1007) at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:747) at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:203) at org.eclipse.jetty.server.HttpConnection.handle(HttpConnection.java:406) at org.eclipse.jetty.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:462) at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:436) at java.lang.Thread.run(Thread.java:662)