I found the error by myself. The set-command somehow converted the note. I passed the container of the note to the event and it works:
Code:
on removeTotal(myContainer)
set myLines to paragraphs of note of myContainer
set myCount to count of myLines
repeat with i from myCount to 1 by -1
if item i of myLines contains mySpentMark then
tell note of myContainer to delete paragraph i
end if
end repeat
end removeTotal
Best,
Ganwell