> and I see nothing wrong in the backtrace posted there.
>  So more details are required to understand what's going on here.

The problem is that, when moving the point to the headers view and, say, 
pressing C-a, Emacs becomes *unresponsive*, even C-g does not work — I 
have to kill it.

/CC Dirk-Jan C. Binnema, the author of mu4e, who may be able to give 
more details.

>  Also, would you mind trying the 25.1 release candidate, to see if the 
>  problem still exists there?

I compiled Emacs from the Git repository and the problem still exists.