assuming you are referring to Jira (Server) I further assume you had an App installed which is now not available anymore - for some reason.
While membersOf() is a stock JQL function of Jira, membersOfGroups() to my latest knowledge is provided by an App, probably J-Tricks.
I would reach out to your Jira administrator if there was J-Tricks in place, and if so, what happened to the App after upgrade.
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