If I understand the problem here, some site-admins are unable to use the log in as another user feature. I have tried to recreate this problem, but so far I have not seen this error. I have tried this on both a paid site as well as a Jira Free site. It seems to be working correctly for me so far.
I found one other support case about this problem and it seems that this was resolved by having the site-admin first login to an incognito/private browser session, and then try to use the log in as user feature there. If this method works for you, then you might have a corrupted cookie/cache in your non-incognito session of your browser that should be cleared in order to resolve this.
If that does not solve this problem, then I would be interested to try to gather more information about this problem so that we can better try to troubleshoot this.
If you are a site-admin of a paid site, it would be best to create a support case about this. You can do that by going to https://support.atlassian.com/contact and select Technical problems & bugs. It might be necessary to grant site access to Atlassian support to investigate this further.
For site-admins of free sites, you won't be able to create a support case by following those steps above as support for free sites is only provided here in Community. That said, I am still interested in trying to better understand more about this problem, such as what Cloud site URL you see this problem on, and perhaps which user account you are trying to login as.
While there we no incidents visible for the days you posted your question probably still there was some kind of hiccup.
Please let us know here in case the error persists.
Somebody from Community can then try to consolidate and ask some other Community members what is probably happening with the login right now.
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