Hi @Ankario ,
Thanks for your reply!
In order to further confirm if the issue is related to Outlook, please configure your account in Mail and check if the issue still exists.
If the issue only occurs in Outlook, in order to avoid the interference of Outlook add-ins and other programs, it's suggested to close other programs temporarily and restart your Outlook in safe mode to check if the issue has any differences (Windows + R> type "outlook.exe /safe">enter).
And when is tested on different computers when same versions of Outlook in some cases they don't behave the same way, adding lines when others don't.
For this, i understand that the same message won't have a white line in Outlook on other computers, right?
If so, please check if there're any difference between the two computers. On the other hand, considering the impact of some third-party programs on your computer, please try to perform a clean boot for windows and check if the issue still occurs.