Mail function and php version

I recently switched from php 7.4. to php 8.0 (with KB version 1.2.23). I use a shared hosting from Host-Europe. When I use the “Send task by email” function, I get an email where the text contains raw HTML that is not parsed.

I found out that the email header contains the FROM: and DATE: twice, e.g.

To: "forname surname " <user@domain.tld>
Subject: TEST
Message-ID: <f7f5a1cc4545ae2c2a88a73f6edf0cf4@assets.domain.tld>
Date: Fri, 09 Sep 2022 12:15:57 +0200
From: via Kanboard <noreply@domain.tld>
Reply-To: user@domain.tld
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Auto-Submitted: auto-generated
From: noreply@domain.tld
Date: Fri, 09 Sep 2022 12:15:57 +0200

When I switch the php version back, everything is fine. Does anyone know the problem?