...
Go to Setup > Notification Service and open the problem Notification task.
Go to Notification E-mais tab > Edit HTML > Home > enable the function Show paragraph marks and other hidden formatting symbols. Then remove an extra space as shown below:
Apply the changes on File tab:
To save the changes, remove the check on the option Task enabled, save the Notification task record, and then set the option back:
Sometimes the above does not help, in this case you can try to copy the affected line from the email body to a Notepad (.txt file) and then copy it back to Notification task email body screen.