Thanks for taking the time to report this!

(In reply to comment #0)

There was also text: "Someone else has changed this page since you started
editing it", but it's not true. Nobody else were not edited the page lately.

Well, maybe that person also canceled editing...

No (because almost no one else is not updating tennis articles in fi-wiki than me), and it happens every time for me when I'm saving pages https://fi.wikipedia.org/wiki/Miesten_kaksinpeli_Yhdysvaltain_avoimessa_tennisturnauksessa_2013 and https://fi.wikipedia.org/wiki/Naisten_kaksinpeli_Yhdysvaltain_avoimessa_tennisturnauksessa_2013

Today when I clicked "save changes" came again "edit conflict", but this time changes were saved.

I uploaded a video about this if it helps: https://commons.wikimedia.org/wiki/File:Edit_conflict_bug_in_Wikipedia.webm

First, page is loading about 2 minutes (my internet may not be the fastest, but not too bad), but at the end of the video when I look at the page history, page has been saved already 1-2 minutes earlier...