Steps to reproduce:

1. Create item such as vfs.file.time["E:\test.file",modify]
2. Attempt to use this item in a Zabbix Calculated item
3. Item will fail with Incorrect function 'now' expression. Check expression part starting from: "vfs.file.time["

Result:
error reason for "MCCMULSVR2:vfs.file.age["E:\FD.BAK",modify]" changed: Incorrect function 'now' expression. Check expression part starting from: "vfs.file.time["

Expected:
Double quotes in an item key should be ignored, so not to interfere with the syntax of a calculated item. If double-quotes must be used (for example, a file path containing spaces), then the Zabbix Calculated item should not be trying to parse the item containing the quotes as if it were the "closing" quotes for the calculated function.

Working Calculated Item: now("vfs.file.time[E:\FD.BAK,modify]")-last("vfs.file.time[E:\FD.BAK,modify]")

Broken Calculated Item: now("vfs.file.time["E:\FD.BAK,modify"]")-last("vfs.file.time["E:\FD.BAK,modify"]")