[imp] Only some messages download and display over ActiveSync on phones

[Sebastian Arcus]

I am facing a really strange issue with Horde/Imp on mobile devices. A 
quick background:

The setup is:

Horde 5.2.21
Lighttpd 1.4.59
PHP 7.3.10
Dovecot 2.3.7.2
Exim 4.92.2

This install has been running for years, and as far as I can think 
nothing has been updated or reconfigured recently - at least not in the 
last few weeks.

In the last few days, one of the users has started to experience some 
emails not downloading on his mobile devices - for no explainable reason.

1. The emails are showing up fine over IMAP in Thuderbird.
2. The mobile devices with problems are two iPhones, one iPad, and I 
just tested it in Bluemail on an Android phone.
3. It isn't always the same messages - if I remove and re-create the 
accounts, I get a different random selection of messages downloaded, 
after which the sync stops. For example I might have most messages from 
today, then yesterday and the day before missing, then some messages 
from the day before. There is nothing logical about it.

I've checked everywhere I can think of:

1. I've forced resyncing of mailboxes in Dovecot (although I don't think 
the problem is there, as all messages show properly in Thunderbird
2. I've changed email passwords
3. I've deleted and re-added email accounts on the iPhone
4. I've hard-reset the iPhones, to try to clear the email cache
5. I've configured the email accounts on an Android phone - still same 
issues
6. I've checked Horde ActiveSync error logs - aside from errors about 
duplicate sync keys, there doesn't seem to be anything else there.
7. I've tried downloading emails over internal wifi and over 4G
8. I've changed sync span on mobiles between 1 day, 3 days, 1 week, 1 
month - the same random behaviour persists

This is happening just to this user, who has two email accounts 
configured on two iPhones and one iPad. All other 10 users are fine. 
Everyone works off the same server, receiving email on iPhones.

I've spent 6 hours on this and frankly I can't make any sense of it. Any 
suggestions would be much appreciated. Thank you.