ckeditor had been updated for H5 (and further has been since then).

In fact, we use the html editor. I forgot about this setting. I 
checked to see if the bug can be reproduces with plain texte but it 
doesn't.

What mislead me is that the bug appeared right after upgrading to H5, 
some weeks ago. The editor worked fine in H4. The only reasons I can 
see for that is either the version of ckeditor used by H4 and H5 is 
not the same or, if this is the same, the interface with H5 is buggy...

So I guess this bug was'nt one. Sorry about this false report.

I'm assuming you mean plaintext input, since you haven't specified 
otherwise (problems with the HTML editor should be reported to the 
ckeditor people)

At a minimum, you need to provide step-by-step instructions as to how 
you can reliably reproduce.

When composing new / answering / transfering message, it is impossible 
ton insert non-ascii characters rightaway. This problem affects all 
users.

However:
- If I clic on the arrow next to spell check and select French 
*sometimes* restores the input of non-ascii characters, but this 
behaviour seems to be random.
- If I add a smily, non-ascii character input starts working and 
continues to work even after removing the smily. This latter strange 
workaround, found by chance by one of our users, works consistently 
for all users.

This problem appeared with the first H5 upgrade.
This is an intranet production server.
The horde installation has been working for more than 3 years now, 
going through H3, H4 and H5.
The system locale is fr_CA.UTF-8.
Users locale are set to default.
Setting my locale to UTF-8 doesnt change then behaviour.