Hi Luke,
I know the question my be self-answered but I just thought I post it here anyways.
The Lookup File Editor app version 3.4.2 seems to be available for Splunk Cloud and actually seems to work for some users, but we found not to be responsive for some others where users cannot save or create new lookups, etc.
Before we have a 3.3.x version of it and it was working flawless.
Is it that the 3.4.2 or newer is still not working properly for Splunk Cloud?
Could you try clearing the cache and seeing if that fixes it? See https://community.splunk.com/t5/Archive/How-do-I-clear-the-cache-to-see-the-changes-after-updating-a...