The Random-by
Farcebook Messenger's "couldn't send" nonsense.
Look it up: it goes back for years, and occurs under various 'circumstances'.
Lots of 'fixes' offered up, which either work or don't for some people.
Why?
Because the 'fixes' are as random as the problem and by pure coincidence happen sometimes when the problem also randomly stops.
In other words an erratic problem that goes away by itself appears to be corrected by whatever action was taken to 'fix' it.
Except that the problem just comes back again. Today, tomorrow, next week, next year.
It's all just another fine example of bloated, bad code.
Or in simple terms "Facebook sucks".