I noticed the error message after expanding the details of a supposedly successful template modification.

Unlinked: Trigger "Foo trigger" on "host1.example.com".
Unlinked: Item "Foo trigger" on "host1.example.com".
Templates "Template bar" unlinked from hosts "host1.example.com".
Created: Item "Foo trigger" on "Template baz".
Created: Item "Foo trigger" on "host2.example.com".
Created: Item "Foo trigger" on "host3.example.com".
[...]
Created: Trigger "Foo trigger" on "Template baz".
Created: Trigger "Foo trigger" on "host2.example.com".
Created: Trigger "Foo trigger" on "host3.example.com".
[...]
Created: Trigger "Foo trigger" on "host123.example.com".
Created: Trigger "Foo trigger" on "host124.example.com".
Created: Trigger "Foo trigger" on "Template 42".
Invalid argument supplied for foreach() [api/classes/CTriggerGeneral.php:240]
Created: Trigger "Foo trigger" on "host125.example.com".
Invalid argument supplied for foreach() [api/classes/CTriggerGeneral.php:240]
Created: Trigger "Foo trigger" on "host126.example.com".
Invalid argument supplied for foreach() [api/classes/CTriggerGeneral.php:240]
[...]

The Template 'Template bar' has one item and one corresponding trigger.
The Template 'Template bar' has been un-linked from host 'host1.example.com' and assigned to template 'Template baz' and another template.
Total count of hosts is ~950.

At a first glance all items and triggers seem to be successfully created (as stated by frontend)