C# Windows Forms Combobox not working correctly

Question

I'm currently working on a backup program. On Friday everything worked without any problems and the project was almost finished.

Today I started Visual Studio without doing anything else. And every combo box I add has the drop-down menu shifted to the right.

I have removed all the code and created new combo boxes, but they are still wrong displayed. I never had this problem before. Maybe it's the Form Designer, I haven't checked it yet. Maybe someone has had this problem before and can help me? I would be very grateful.

public Form1()
{
   InitializeComponent();
   this.CenterToScreen();
   this.MaximizeBox = false;
   this.FormBorderStyle = FormBorderStyle.FixedSingle;
   this.Font = new Font("Arial", 10, FontStyle.Regular);
   this.PathSettingsPanel.BackColor = System.Drawing.Color.LightGray;
   this.UsernameComboboxCreate.DropDownStyle = ComboBoxStyle.DropDownList;
   this.SelectBackupSend.DropDownStyle = ComboBoxStyle.DropDownList;
   this.DtDUsernameSource.DropDownStyle = ComboBoxStyle.DropDownList;
   this.DtDUsernameTarget.DropDownStyle = ComboBoxStyle.DropDownList;
   this.indicator.Text = string.Empty;
   this.SendBackupIndicator.Text = string.Empty;
   this.IndicatorSource.Text = string.Empty;
   this.IndicatorTarget.Text = string.Empty;
   this.panel3.BorderStyle = BorderStyle.FixedSingle;
   this.panel4.BorderStyle = BorderStyle.FixedSingle;
   this.panel5.BorderStyle = BorderStyle.FixedSingle;
   this.ServerBackups.View = View.Details;
   this.Updating.Visible = true;
   this.ServerBackups.UseCellFormatEvents = true;
   Serverpath.Text = Main.ReadPathFromFile("server");
   PopulateListBoxes();
   StartCopy.DoWork += backgroundWorker1_DoWork;
   StartCopy.RunWorkerCompleted += backgroundWorker1_RunWorkerCompleted;
   StartCopy.WorkerReportsProgress = true;
   StartCopy.WorkerSupportsCancellation = true;
   FillProgressBar.DoWork += backgroundWorker2_DoWork;
   FillProgressBar.RunWorkerCompleted += 
   backgroundWorker2_RunWorkerCompleted;
   FillProgressBar.ProgressChanged += 
   backgroundWorker2_ProgressChanged;
   FillProgressBar.WorkerReportsProgress = true;
   FillProgressBar.WorkerSupportsCancellation = true;
 }

Edit

I've tried a little around. The problem persists even if I create a new project and copy code and forms.

When I create a combobox outside the tab control, it works as it should. Once I click a combobox inside the tab control it will be displayed incorrectly. After that the box which works at the beginning doesn't work anymore.

Okay the problems seems to be the tabcontrol. I just have no idea why.