Modal input elements have same id attribute as elements on main page.
Found conflicts on pages:
1. Trigger edit form, expression constructor modal.
2. Monitoring overview, application selection modal group filter have same id as monitoring overview page group selector.
3. Checkboxes that have an additional hidden input field with same ID. For example "Show hosts in maintenance" and other few.
There could be more affected pages, detailed check is required.
Id's on popups should be different from id's on main page. Be careful, because some pages have several popups simultaneously.