Steps to reproduce:

Click on Configuration, Host groups, click on group named Templates. On the left panel we see various templates in this group. We try to move one item or some items or all of them into another group.(For our internal housekeeping). It does not matter how many we try to move, when moving and then clicking on Save, we got the error message mentioned in subject.

When we do the same thing on our custom group name, for example group called Databases, then everything works just fine.
So it is related to group named Templates, which is built in? And the error and action is similar to that one mentioned in bug ZBX-1479...

To make things even stranger, we can do this by another way - I can click on hostgroup member, for example in group Templates is a member called Template_something. So when we click on that member,
then we can select it into another group just fine. But this is doing one-by-one and when you have a lot of members, it is not an option.