Trying via the api on the beta cluster:

{
    "error": {
        "code": "internal_api_error_Error",
        "info": "[XtAeOqwQBHcAAH9Vo68AAAAR] Exception caught: Cannot use object of type User as array",
        "errorclass": "Error",
        "trace": "Error at /srv/mediawiki/php-master/extensions/CentralAuth/includes/specials/SpecialGlobalGroupMembership.php(107)\n#0 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(147): SpecialGlobalGroupMembership->fetchUser(User)\n#1 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(103): ApiUserrights->getUrUser(array)\n#2 /srv/mediawiki/php-master/includes/api/ApiMain.php(1583): ApiUserrights->execute()\n#3 /srv/mediawiki/php-master/includes/api/ApiMain.php(523): ApiMain->executeAction()\n#4 /srv/mediawiki/php-master/includes/api/ApiMain.php(494): ApiMain->executeActionWithErrorHandling()\n#5 /srv/mediawiki/php-master/api.php(84): ApiMain->execute()\n#6 /srv/mediawiki/w/api.php(3): require(string)\n#7 {main}"
    },
    "servedby": "deployment-mediawiki-07"
}

Error at /srv/mediawiki/php-master/extensions/CentralAuth/includes/specials/SpecialGlobalGroupMembership.php(107)
#0 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(147): SpecialGlobalGroupMembership->fetchUser(User)
#1 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(103): ApiUserrights->getUrUser(array)
#2 /srv/mediawiki/php-master/includes/api/ApiMain.php(1583): ApiUserrights->execute()
#3 /srv/mediawiki/php-master/includes/api/ApiMain.php(523): ApiMain->executeAction()
#4 /srv/mediawiki/php-master/includes/api/ApiMain.php(494): ApiMain->executeActionWithErrorHandling()
#5 /srv/mediawiki/php-master/api.php(84): ApiMain->execute()
#6 /srv/mediawiki/w/api.php(3): require(string)
#7 {main}

That being said, I believe the issue is unrelated the the error above. UserrightsPage::changeableGroups and some other methods were made private (had no visibility), and SpecialGlobalGroupMembership tries to override them

Change 599432 had a related patch set uploaded (by DannyS712; owner: DannyS712):
[mediawiki/core@master] UserrightsPage: Restore public visibility

https://gerrit.wikimedia.org/r/599432

SpecialUserRights->fetchUser and SpecialGlobalGroupMembership->fetchUser both expect a string username.

But:

#0 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(147): SpecialGlobalGroupMembership->fetchUser(User)
#1 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(103): ApiUserrights->getUrUser(array)

.. indicates that ApiUserrights->getUrUser is sending down a User object instead. Where does ApiUserrights get it from? Did that change recently?

In T253909#6174381, @Krinkle wrote:

SpecialUserRights->fetchUser and SpecialGlobalGroupMembership->fetchUser both expect a string username.

But:

#0 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(147): SpecialGlobalGroupMembership->fetchUser(User)
#1 /srv/mediawiki/php-master/includes/api/ApiUserrights.php(103): ApiUserrights->getUrUser(array)

.. indicates that ApiUserrights->getUrUser is sending down a User object instead. Where does ApiUserrights get it from? Did that change recently?

Couldn't find anything that changed that recently, but is it possible that the bug was there previously and wasn't seen?
getUrUser should be sending $params['user'] ?? '#' . $params['userid']; which is a string

@Urbanecm What is the error exactly? Or is there no program error?

In T253909#6174390, @DannyS712 wrote:

In T253909#6174381, @Krinkle wrote:

[…]

.. indicates that ApiUserrights->getUrUser is sending down a User object instead. Where does ApiUserrights get it from? Did that change recently?

Couldn't find anything that changed that recently, but is it possible that the bug was there previously and wasn't seen?
getUrUser should be sending $params['user'] ?? '#' . $params['userid']; which is a string

It isn't a string, but I agree that it should be indeed. This might explain why:

			'user' => [
				ApiBase::PARAM_TYPE => 'user',
				¬
				UserDef::PARAM_RETURN_OBJECT => true,

Anyway, I suggest we move this to a different ticket as this error is not happening in production and possibly not related to the current report.

In T253909#6174400, @Krinkle wrote:

@Urbanecm What is the error exactly? Or is there no program error?

There is no exception, it just claims I can't change the rights. Screenshot is in description. Danny's patch should fix this (thanks), althrough setting UserrightsPage::changeableGroups to protected should be enough for this particular bug.

Change 599438 had a related patch set uploaded (by Reedy; owner: DannyS712):
[mediawiki/core@wmf/1.35.0-wmf.34] UserrightsPage: Restore visibility (previously implicitely public)

https://gerrit.wikimedia.org/r/599438

Change 599438 merged by jenkins-bot:
[mediawiki/core@wmf/1.35.0-wmf.34] UserrightsPage: Restore visibility (previously implicitely public)

https://gerrit.wikimedia.org/r/599438

Change 599432 merged by jenkins-bot:
[mediawiki/core@master] UserrightsPage: Restore visibility (previously implicitely public)

https://gerrit.wikimedia.org/r/599432

Mentioned in SAL (#wikimedia-operations) [2020-05-28T21:12:17Z] <jforrester@deploy1001> Synchronized php-1.35.0-wmf.34/includes/specials/SpecialUserrights.php: T253909 Restore visibility (previously implicitely public) (duration: 01m 06s)

Thanks everyone