Personal Stationary HTML Themes suddenly stopped working for some users in Outlook (File > Options > Mail > Stationary and Fonts > Theme). People who had their themes selected from before and didn't touch the settings were not affected. However, as soon as anyone tried to change the theme, it would stop working for them. The Stationary and Signatures window in Outlook settings would show the correct theme, but when starting a new email the theme would not load. Instead, Outlook just started with a new blank email.
The issue was caused by a Microsoft Office bug introduced in version 2310. The short-term solution was to either downgrade MS Office to version 2309, or set up .oft Outlook Template files for users to compose new emails.
The issue was resolved by Microsoft in MS Office 365 version 2311 released at the end of November.
October - November 2023
Windows 10 Pro
MS Office 365