Strangely, the e-mail problem has reapparead after a couple of days, reason unknown! Even resetting the mail app is therefore only a temporary solution. I hope that the problem will be fixed in the next update. As a workaround, I still have K9 installed, which works as expected. (So there’s hope that the mail app will be fixed because it is based on K9 
)
EDIT: Well, I sent an e-mail for the first time after resetting the app, maybe this caused the problem? I don’t know.