[kronolith] Call to a member function write() on null

[krono]

Hello kronolith list,

I have an installation of horde using
bitnami-horde-5.2.14-1-linux-x64-installer.run on ubuntu 16.04.
As for the custom mail server I toggle between 2 almost identical setups:
1. dovecot/postfix on ubuntu 12.04
2. dovecot/postfix on ubuntu 16.04
The toggling is done by updating the hostspec and host in imap section of
/opt/horde-5.2.14-1/apps/horde/htdocs/imp/config/backends.php and
restarting the stack - everything works fine.

I was creating an event in the calendar of a user while connected to the
mail server of ubuntu 12.04.
When I update the horde backend to the ubuntu 16.04 installation, and
try to login, I get the following web message:

A fatal error has occurred
Call to a member function write() on null
Details have been logged for the administrator.

and a message pops up on the console:
HORDE[15202]: [imp] Call to a member function write() on null [pid 15202
on line 4429 of
"/opt/horde-5.2.14-1/php/lib/php/Horde/Imap/Client/Socket.php"]

This happens only when I try to login to that specific user while
connected to the 16.04.
No problem when the backend points to the 12.04 and no problem to login
to other users when the backend points to 16.04. I also created a
calendar for another user on the 16.04, just to verify that 16.04 is
capable of the calendar feature, and it works fine.

The mail setups are identical - same usernames, same passwords, and same
emails inside (copied manually from 12.04 to 16.04). Also the
permissions on the mail files are identical.
The problem appeared right after adding this calendar event.

It seems that for the problematic user, some data is kept somewhere, but
I don't know where.

What I tried until now:
1. erase the event on 12.04 - still error when 16.04 is connected.
2. reinstall the horde stack - same.
3. look for my event name string inside the horde stack and erase every
mention.
4. delete horde caches on /tmp
5. look inside the mysql database and search for the event.

checking further in the code:
https://github.com/horde/horde/blob/master/framework/Imap_Client/lib/Horde/Imap/Client/Socket.php#L4429

Any idea what it can be?
Where can I find the list of locations that kronolith/horde keeps for
each user? for the kronolith? for login?

Thanks!
krono