Shosaku Takatani 

Can you give us some more details about this issue?
What is the filetype of the document you are working on? 
When the error shows up again, please click on the blue floppy icon on the left bottom of the error message window. This will save a file with the extension "sdlerror.xml". Then attach this file to your post. 
This will help us to narrow down the problem.

Thank you for your reply, Blaser-san!
It's a great help.

The file type is Word (.docx).

I'm not entirely sure, but the error seems to occur only when I'm overwriting existing text.

Here is the error log:

 <SDLErrorDetails time="2024/09/22 2:59:44">
<ErrorMessage>Parameter is not valid.</ErrorMessage>
<Exception>
<Type>System.ArgumentException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
<ParamName />
<HelpLink />
<Source>System.Drawing</Source>
<HResult>-2147024809</HResult>
<StackTrace><![CDATA[ at System.Drawing.Font.GetHeight(Graphics graphics)
at System.Drawing.Font.GetHeight()
at System.Drawing.Font.get_Height()
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.AbstractSuggestionWindow.SetLocation()
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.SuggestionWindow..ctor(ISuggestionDataProvider suggestionDataProvider, List`1 suggestionData, Form parentForm, SideBySideEditor editor, SuggestionSite site, Boolean isInternal, SuggestionMode mode, ITranslatableDocumentEditorSettings settings, Int32 imeBehavior)
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.SuggestionWindow.ShowSuggestionWindow(Form parent, ISideBySideDocumentEditingView view, SideBySideEditor editor, ISuggestionDataProvider suggestionDataProvider, SuggestionSite site, Char firstChar, Int32 sourceStartPosition, SuggestionMode mode, ITranslatableDocumentEditorSettings settings, Boolean requireCaseResolving, Boolean incrementTextOffset, Int32 imeBehavior)
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.IntellisenseEditing.ShowSuggestionWindow(Form parent, ISideBySideDocumentEditingView view, SideBySideEditor editor, ISuggestionDataProvider suggestionDataProvider, SuggestionSite site, Char firstChar, Int32 sourceStartPosition, SuggestionMode mode, ITranslatableDocumentEditorSettings settings, Boolean requireCaseResolving, Boolean incrementTextOffset)
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.IntellisenseEditing.LaunchIntellisenseWindow(Boolean keyPressed, Char lastChar, Boolean afterImeCompositionUpdate)
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.IntellisenseEditing.ProcessDocumentContentChanged(List`1 operations, Boolean afterImeCompositionUpdate)
at Sdl.TranslationStudio.Editor.TranslationEditor.Intellisense.IntellisenseEditing.Document_ContentChanged(Object sender, FrameworkDocumentContentChangedEventArgs e)
at System.EventHandler`1.Invoke(Object sender, TEventArgs e)
at AbstractFrameworkDocument.raise_ContentChanged(Object value0, FrameworkDocumentContentChangedEventArgs value1)
at AbstractFrameworkDocument.OnContentChanged(Object sender, FrameworkDocumentContentChangedEventArgs args)
at FrameworkDocumentEventSource.RaiseContentChanged(FrameworkDocumentEventSource* )
at FrameworkDocumentEventSource.vv_AfterCompositeCommandCompleted(FrameworkDocumentEventSource* )
at Tree.DocumentControl.vl_WindowProc(DocumentControl* , HWND__* , UInt32 , UInt32 , Int32 )
at FrameworkDocumentControl.vl_WindowProc(FrameworkDocumentControl* , HWND__* h_wnd, UInt32 n_msg, UInt32 w_p, Int32 l_p)
at System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at System.Windows.Forms.NativeWindow.DefWndProc(Message& m)
at System.Windows.Forms.Control.DefWndProc(Message& m)
at System.Windows.Forms.Control.WmKeyChar(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)]]></StackTrace>
</Exception>
<Environment>
<ProductName>Trados Studio</ProductName>
<ProductVersion>Studio18</ProductVersion>
<EntryAssemblyFileVersion>18.0.0.1013</EntryAssemblyFileVersion>
<OperatingSystem>Microsoft Windows 11 Home</OperatingSystem>
<ServicePack>NULL</ServicePack>
<OperatingSystemLanguage>1041</OperatingSystemLanguage>
<CodePage>932</CodePage>
<LoggedOnUser>DESKTOP-H6SQH2P\shota</LoggedOnUser>
<DotNetFrameWork>4.0.30319.42000</DotNetFrameWork>
<ComputerName>DESKTOP-H6SQH2P</ComputerName>
<ConnectedToNetwork>True</ConnectedToNetwork>
<PhysicalMemory>16115192 MB</PhysicalMemory>
</Environment>
</SDLErrorDetails>

Please ignore this comment: "The error seems to occur only when I'm overwriting existing text."

I've found that the error can occur even when I'm simply typing text without overwriting any existing text.

Shosaku Takatani 

Have you tried to do a RESET of Studio following the steps described in this KB article:
Reset Trados Studio to its default state (sdl.com)

Do you have any plugins installed?
If yes, try to remove them temporarily to see if the problem disappears.

Thank you for your reply.

I happened to find the following thread, where someone asked a question about the same issue:
https://community.rws.com/product-groups/trados-portfolio/trados-studio/f/studio/20709/repeated-error-message-parameter-not-valid

Based on the information in that thread, I reset the font for Japanese from "Arial" to the default setting, and that seems to have resolved the problem!

I'll try the workaround you suggested if the error occurs again.

Thank you!