@ClausR yes, that was interpreted correctly. Sometimes it is difficult to find the right words.
In the meantime, I know a bit more about my problem. I deleted an email, but later I realised that I still wanted to reply. So I restored the email by clicking on the bin icon. When I deleted the same e-mail again later, it was only shown under "all messages" in the Trash folder, but not under "all accounts" in the Trash folder.
I don't know why it happens this way, but all emails I delete for the first time show up in both Trash folders.
At the moment I only use my Vivaldi mail-account with Vivaldi-mail, so it's not a provider-issue.