A paper seems to have completely disappeared from editor's hands I have submitted a paper to a journal on June 2017. The corresponding author is my coauthor. The first response of the editor was after a year, June 2018:

Your paper has been sent, consecutively, to four referees.
  Of the preceding three, one declined to review it and two 
  never responded to the invitation or to multiple reminders.
  The fourth one accepted; a report is expected in September [2018]. 

Since then, the editor disappeared. No notice at all, neither in the positive, nor in the negative. Obviously we tried to contact him again multiple times (approximately every two-three months) through the journal's platform. No response at all. We sent countless emails to the editor, either via the platform or through his personal email (my coauthor knows him in person). No response at all. We tried to contact the chief editor. No response at all.
In a few days, it will be exactly two years since the submission: this is an incredible amount of time, especially since we absolutely don't know what is the motivation for this delay.
What shall we do?
[I'm not posting this on academia, since this is a paper in Mathematics; but feel free to migrate the discussion elsewhere if you feel like so]
Unfortunately I am unable to comment as I lack sufficient reputation. 


*

*Yes, "consecutively" means exactly that 3 people in a row refused to referee the paper; the fourth accepted and then disappeared since June 2018.

*The editor-in-chief should be aware of what's going on, more so because we wrote him an email two weeks ago or so. No answer.

*The journal is a pretty good and reputable one. At least until I spread this voice.

*My coauthor has nothing against me if I reveal the name of the journal: it's JoA https://www.journals.elsevier.com/journal-of-algebra
 A: Here are some additional things to consider:


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*Sometimes email addresses are blocked, for one reason or another.  One of my coauthors had his institution block all hotmail addresses, which led to some trouble in our communications.  In your post you mention "countless" emails; this can cause certain servers to start blocking emails.  Conversely, their emails to you might be blocked.  In your correspondence you can mention that you haven't heard anything, and perhaps give them your phone number.

*Sometimes people are away from computers for multiple weeks.  Then it takes a while to contact the referee.  My personal rule of thumb is to contact an editor and if I receive no reply then I follow up a week later.  I repeat this one more time.  After 3 emails (and 3 weeks of waiting), I believe it is then time to contact the editor-in-chief.  During this process you might try to use alternate email addresses which haven't been potentially blocked.  You do not want to pester an editor with "countless" emails.

*Some of the issues above can be avoided if the journal in question has its own way to contact editors, which it appears you already tried to use.

*Nobody should wait multiple months between trying to contact an editor, if they are receiving no responses.  That is unreasonably long.  This noncommunication issue should have been dealt with near the date the editor gave for the report to be done.
Given the time-frame, if you can't get a response from the editor-in-chief through the methods people gave in the comments, I would send one final (kindly worded!) email saying that if you don't hear from them within two weeks, then your paper should be considered as withdrawn.
