After the upgrade you may experience a problem with deleting hosts. It is caused by configured maintenances having meanwhile all hosts deleted. To resolve this issue, go to Configuration → Maintenance, select 'All' in the Group dropdown and then delete all maintenances without host groups/hosts or add host groups/hosts in them.
Note that while previously it was allowed to delete the last host in a defined maintenance, now it is disallowed.
If you have "orphaned" problems (with no item/host) displayed in the problem view, caused by an issue in Zabbix 4.0.3, they should be removed manually by deleting the corresponding triggers - for example, in the Problems view click on the problem name for the orphaned problem, select Configuration from the context menu and then, in the opened trigger edit form, click on Delete. Alternatively the corresponding triggers can be found and deleted in the list of all triggers.
Previously, if a system had devices with dynamically changing paths or if a device got manually removed, Zabbix agent would not release them and would keep the connection resulting in collector overfilling. Now Zabbix agent will terminate “stale” device connections if the values of corresponding items are not accessed for more than 3 hours. Note that the corresponding items (
vfs.dev.write), if having the update interval longer than 3 hours, will now always return '0'.