warning messages on 'latest data' page

[cybermcm]

I noticed warnings/error messages on top of my screen on the 'latest data' page

• date() expects parameter 2 to be int, string given [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() → date() in include/func.inc.php:253]
• date() expects parameter 2 to be int, string given [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() → date() in include/func.inc.php:254]
• Undefined offset: 0 [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() in include/func.inc.php:254]
• date() expects parameter 2 to be int, string given [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() → date() in include/func.inc.php:255]
• date() expects parameter 2 to be int, string given [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() → date() in include/func.inc.php:256]
• Undefined offset: 0 [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() in include/func.inc.php:256]
• date() expects parameter 2 to be int, string given [zabbix.php:22 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → CPartial->getOutput() → include() → formatHistoryValue() → convertUnits() → zbx_date2str() → date() in include/func.inc.php:275]

I have 5 host groups and this happens only with the host group 'Zabbix servers' which is currently only one server. All other host groups do not show these messages.
Everything seems to work though...

I'm using Zabbix with docker:

zabbix/zabbix-server-mysql:alpine-latest
zabbix/zabbix-agent2:alpine-latest
HOST (Raspberry OS): Linux s01 5.10.60-v7l+ #1449 SMP Wed Aug 25 15:00:44 BST 2021 armv7l GNU/Linux
DB 10.5.12-MariaDB

Any idea how to fix this? Happy to provide more info if needed...