If you see this message when creating a new email in classic Outlook for Windows, Outlook is usually trying to load an item used in an HTML email – most often a theme, stationery file, signature image or background – that has moved or is no longer available.

The error can be unsettling, but it does not normally mean that your email account or messages have been lost.

This is the most likely solution, particularly if the error appears when you start a new message.

1. Open Outlook (classic).
2. Select File, then Options.
3. Select Mail.
4. Under Compose messages, select Stationery and Fonts.
5. On the Personal Stationery tab, select Theme.
6. Choose (No Theme).
7. Select OK on each open window.
8. Close Outlook completely, reopen it, then try creating a new email.

Changing the theme back to (No Theme) resolved the issue for the user who raised it in Microsoft’s Q&A forum.

If the message is still appearing, check whether your signature includes images or links that are stored in OneDrive, SharePoint, a network drive, or another location you may no longer be able to access.

1. In Outlook, go to File > Options > Mail > Signatures.
2. Select each signature and look for images, logos or links.
3. Remove or replace anything that points to a shared or cloud location that may have changed.
4. Save the signature and try a new email again.

Try these checks one at a time. Test a new email after each step, so you know which change fixed it.

• Start Outlook in Safe Mode: press Windows key + R, type outlook.exe /safe, then select OK. If the problem disappears, an Outlook add-in may be responsible.
• Disable add-ins temporarily: go to File > Options > Add-ins. At the bottom, choose COM Add-ins and select Go. Untick the add-ins, restart Outlook and test. Re-enable them one at a time afterwards.
• Reset custom stationery: press Windows key + R, enter %APPDATA%\Microsoft\Stationery, then rename the folder to Stationery-old. Outlook will not use the old stationery while you test. Do not delete the folder unless you are sure you no longer need it.
• Repair Microsoft 365: use Control Panel > Programs and Features, select Microsoft 365 Apps, then Change. Start with Quick Repair; if that does not help, use Online Repair.

Source: Microsoft Q&A – Outlook Classic error when opening new emails.