I forgot to say that it is only when the widget is in a sub layout (or sub-widget with an horizontal layout). Like when I have few widgets on the same line in a parent layout, the text won't show up.

Qt Code: 
  1. groupBox = new QGroupBox(LogInScreen);
  2.         groupBox->setObjectName(QString::fromUtf8("groupBox"));
  3.         groupBoxLayout = new QVBoxLayout(groupBox);
  4.         groupBoxLayout->setObjectName(QString::fromUtf8("groupBoxLayout"));
  5.         titleLabel = new QLabel(groupBox);
  6.         titleLabel->setObjectName(QString::fromUtf8("titleLabel"));
  7.         titleLabel->setAlignment(Qt::AlignCenter);
  8.  
  9.         groupBoxLayout->addWidget(titleLabel);
  10.  
  11.         subTitleLabel = new QLabel(groupBox);
  12.         subTitleLabel->setObjectName(QString::fromUtf8("subTitleLabel"));
  13.         subTitleLabel->setAlignment(Qt::AlignCenter);
  14.  
  15.         groupBoxLayout->addWidget(subTitleLabel);
  16.  
  17.         middleSpacer = new QSpacerItem(20, 40, QSizePolicy::Minimum, QSizePolicy::Expanding);
  18.  
  19.         groupBoxLayout->addItem(middleSpacer);
  20.  
  21.         userInfoLayout = new QHBoxLayout();
  22.         userInfoLayout->setSpacing(25);
  23.         userInfoLayout->setObjectName(QString::fromUtf8("userInfoLayout"));
  24.         userInfoLayout->setContentsMargins(0, -1, 0, -1);
  25.         leftlSpacer = new QSpacerItem(40, 20, QSizePolicy::Expanding, QSizePolicy::Minimum);
  26.  
  27.         userInfoLayout->addItem(leftlSpacer);
  28.  
  29.         loginPasswordLayout = new QFormLayout();
  30.         loginPasswordLayout->setObjectName(QString::fromUtf8("loginPasswordLayout"));
  31.         loginPasswordLayout->setFieldGrowthPolicy(QFormLayout::AllNonFixedFieldsGrow);
  32.         loginPasswordLayout->setVerticalSpacing(15);
  33.         loginLabel = new QLabel(groupBox);
  34.         loginLabel->setObjectName(QString::fromUtf8("loginLabel"));
  35.  
  36.         loginPasswordLayout->setWidget(0, QFormLayout::LabelRole, loginLabel);
  37.  
  38.         loginLineEdit = new QLineEdit(groupBox);
  39.         loginLineEdit->setObjectName(QString::fromUtf8("loginLineEdit"));
  40.  
  41.         loginPasswordLayout->setWidget(0, QFormLayout::FieldRole, loginLineEdit);
  42.  
  43.         passwordLabel = new QLabel(groupBox);
  44.         passwordLabel->setObjectName(QString::fromUtf8("passwordLabel"));
  45.  
  46.         loginPasswordLayout->setWidget(1, QFormLayout::LabelRole, passwordLabel);
  47.  
  48.         passwordLineEdit = new QLineEdit(groupBox);
  49.         passwordLineEdit->setObjectName(QString::fromUtf8("passwordLineEdit"));
  50.         passwordLineEdit->setEchoMode(QLineEdit::Password);
  51.  
  52.         loginPasswordLayout->setWidget(1, QFormLayout::FieldRole, passwordLineEdit);
  53.  
  54.  
  55.         userInfoLayout->addLayout(loginPasswordLayout);
  56.  
  57.         forgotSomethingLayout = new QVBoxLayout();
  58.         forgotSomethingLayout->setSpacing(15);
  59.         forgotSomethingLayout->setObjectName(QString::fromUtf8("forgotSomethingLayout"));
  60.         forgotLoginLabel = new QLabel(groupBox);
  61.         forgotLoginLabel->setObjectName(QString::fromUtf8("forgotLoginLabel"));
  62.         forgotLoginLabel->setOpenExternalLinks(true);
  63.  
  64.         forgotSomethingLayout->addWidget(forgotLoginLabel);
  65.  
  66.         forgotPasswordLabel = new QLabel(groupBox);
  67.         forgotPasswordLabel->setObjectName(QString::fromUtf8("forgotPasswordLabel"));
  68.         forgotPasswordLabel->setOpenExternalLinks(true);
  69.  
  70.         forgotSomethingLayout->addWidget(forgotPasswordLabel);
  71.  
  72.  
  73.         userInfoLayout->addLayout(forgotSomethingLayout);
  74.  
  75.         rightSpacer = new QSpacerItem(40, 20, QSizePolicy::Expanding, QSizePolicy::Minimum);
  76.  
  77.         userInfoLayout->addItem(rightSpacer);
  78.  
  79.  
  80.         groupBoxLayout->addLayout(userInfoLayout);
  81.  
  82.         middleSpacer_2 = new QSpacerItem(20, 40, QSizePolicy::Minimum, QSizePolicy::Expanding);
  83.  
  84.         groupBoxLayout->addItem(middleSpacer_2);
  85.  
  86.         buttonsLayout = new QHBoxLayout();
  87.         buttonsLayout->setObjectName(QString::fromUtf8("buttonsLayout"));
  88.         buttonsLeftSpacer = new QSpacerItem(40, 20, QSizePolicy::Expanding, QSizePolicy::Minimum);
  89.  
  90.         buttonsLayout->addItem(buttonsLeftSpacer);
  91.  
  92.         okButton = new QPushButton(groupBox);
  93.         okButton->setObjectName(QString::fromUtf8("okButton"));
  94.         QSizePolicy sizePolicy(QSizePolicy::Maximum, QSizePolicy::Fixed);
  95.         sizePolicy.setHorizontalStretch(0);
  96.         sizePolicy.setVerticalStretch(0);
  97.         sizePolicy.setHeightForWidth(okButton->sizePolicy().hasHeightForWidth());
  98.         okButton->setSizePolicy(sizePolicy);
  99.  
  100.         buttonsLayout->addWidget(okButton);
  101.  
  102.         cancelButton = new QPushButton(groupBox);
  103.         cancelButton->setObjectName(QString::fromUtf8("cancelButton"));
  104.         sizePolicy.setHeightForWidth(cancelButton->sizePolicy().hasHeightForWidth());
  105.         cancelButton->setSizePolicy(sizePolicy);
  106.  
  107.         buttonsLayout->addWidget(cancelButton);
  108.  
  109.  
  110.         groupBoxLayout->addLayout(buttonsLayout);
  111.  
  112.  
  113.         mainLayout->addWidget(groupBox);
  114.  
  115.         bottomSpacer = new QSpacerItem(20, 40, QSizePolicy::Minimum, QSizePolicy::Expanding);
  116.  
  117.         mainLayout->addItem(bottomSpacer);
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If needed I can upload a screenshot of the bug.

Thanks for your help!
Boris