Use Case:- We have created three custom group which are present in one group ty

If you call the user() function with some sample data by itself in the expressions editor do you get the same error?
Could you attach your application-server.log?

No. with same data its working fine. I am able to get list of users (userList attachment ) but when tried to show first name of user in grid second column got error.

serverlogs.txt

filtered server log

serverlogs.txt

The most recent errors I see in the logs have the following issue:

Caused by: com.appiancorp.suiteapi.common.exceptions.InvalidUserException: [InvalidUserException]

It looks like the user you are passing in is invalid. Are you storing the usernames as Text, or as User datatype?

Try wrapping the ri!username in the touser() function, which would give you
user(touser(ri!username), "firstName")

i am storing it as text .. tried touser() also but it doesnt work

i have validated each username separately against firstname attributes and didnt encounter any invalid user error

Could you provide a screenshot of the error you are seeing as well as a screenshot of the rule getFirstName?

attached..

I did a simple test of this in my environment and was only able to recreate that error when the username did not exist in the system.

Can you confirm that none of the users have been deactivated?

Could you try testing by replacing fn!apply(rule!getFirstName(_),local!getUniqueList)) with a hardcoded list of users, something like fn!apply(rule!getFirstName(_),{"user1", "user2", "user3"}))

removed all other users expect my user. but still getting same error.. If i pass static array it works always..