There seems to be something wrong with the flagging.
When trying to flag this question I got an error. Reloading the page didn’t fix the issue (so it’s not a browser caching problem).
When I click either flag or close and submit that with an according close reason, the error shown in the screenshot pops up. Reloading the page shows the close/flag votes are counted dispite the error message.
Looking at the network infos, it shows the error is the result of a HTTP ERROR CODE 500.
As the next screenshot shows, I was able to increase that counter… that’s 9 x one of my votes there! (Reloading the page indicates the server increased the according counter.)
Funny enough, the question is still open after it got 9 close-votes and flags… from a moderator.
Since SE just recently switched to HTTPS, I’m assuming this is a server-side problem which might be related to that. If it isn’t, another server-side problem might need a fix.
Note that the problem only occurs with this “migrated” question. A few minutes before, I was able to flag Q 43123 as a duplicate without any issues.
Is the problem known already? If not, please regard this to be a bug report.