If type of an existing item without timeout (example: Zabbix trapper) was changed to an item type, that has timeout (example: script), the value of the timeout is missing (can be observed when inspecting the page in DevTools), and testing the new item will result in "Unsupported timeout value" error message.
Can be reproduced by editing or cloning the existing item, and changing the type.

Steps to reproduce:
Precondition: item without timeout was previously created (Zabbix trapper or similar)

1. Open Data collection -> Hosts -> Items
2. Click on the existing trapper item
3. Change the type of the item to the type, that has the Timeout parameter, example: Script
4. Add necessary data, without changing the value of the Timeout
5. Click test
6. Click Get value and test

Result:
"Unsupported timeout value" error message is displayed.

See video: 

See screenshot:

Expected:
Value of the timeout should be present, testing of the new item - successful.