Using classic Outlook for Windows in business environments
Since New Email works normally when stationery is disabled, that does seem to narrow it down to the stationery feature rather than your mailbox or Outlook profile.
I’d first check the exact Outlook version/build under File > Office Account > About Outlook and note when the issue started. If this began right after an Office update, that would be useful to report along with the build number.
As a temporary workaround, you can leave stationery set to No stationery so New Email works normally.
I’d also try Outlook in Safe Mode (outlook.exe /safe). If stationery still causes the error there, that would make an add-in less likely and point more toward Outlook itself.
Since all built-in themes are affected, I’d report it through Help > Feedback with the Outlook build and the name of the stationery used.