Steps to reproduce:

1. Create template with a few items.
2. Assign template to large number of hosts (500+)
3. Make changes to Template linked to hosts
4. If large operation (i.e. add Tag to 10 Triggers within said Template, applied to 500 hosts)
5. Zabbix screen will refresh, but with blank main window

Result:
Screenshot 1 - Mass update on mutiple tags within Template

Screenshot 2 - Add Central as Tag to all 7 triggers

Screenshot 3 - Blank page after 60 seconds of loading (even debug menu not showing!)

Triggers are NOT updated with Tags following this process.

Any other operation of this kind (add macro to Item check within a template linked to multiple hosts for example...)
Expected:

For the items to be updated as requested and be taken back to the mains screen with 'Request processed successly' message at top.