i suggest setting View>headers>normal and see if that corrects the issue.

Thank you for your suggestion, David. I am sorry, it did not help. But I noticed something interesting. The headers in the mails that I get in HTML turn out to be empty. And when under View I look up the message source this looks different. It shows a lot more code.

Yes, view source shows the actual HTML code. Not usually needed. Since you set headers to normal, I don't know what to advise, other than possible closing and restarting thunderbird. Maybe someone else here has specific instructions to fix viewing the headers on messages.

I am still getting this problem once in a while. The problem remains for the particular e-mail after closing and restarting Thunderbird. But it does show correctly in Ziggo webmail.