[horde] Sudden user-interface changes for some users

[DrOteonu]

Dear Horde community,
We have been using Horde for years and with pleasure. However,
suddenly, in some accounts the interface has changed, in a very
unpleasant way. The strange thing is that this only happened in some
user-accounts, without these users having made any changes in their
settings. And without us having upgraded Horde.
Some examples of the changes:
A. Calendar interface changes:
Original interface:
The URL generated by horde: 
https://[domain]/kronolith/#week%3A20210207Screenshot: https://pasteboa
rd.co/JOiRM7O.png
Among other things:1) It has a "search" field in the upper-right
corner, that functions very well. 2) If you click somewhere in a
column, a dialogue box appears to add a new event. 3) Clicking an
event, makes a user-friendly dialogue box appear that allows you to
edit it.
The changed interface:
The URL generated by horde: 
https://[domain]/kronolith/week.phpScreenshot: https://pasteboard.co/JO
iR9KB.pngAll the features 1, 2 and 3 are gone. It is hard to work with
this interface.
B.  Email client change
The email client changed from a full interface to a minimal hard-to-use 
text-based interface for some users.
(End of examples.)
I have tried to go through all settings numerous times, but to no
avail.
This is just a sample of the things that have changed. Throughout
horde, more things seem to have changed. Funnily enough, this is the
case only for some users. I have the impression that somehow it serves
other dynamically-generated webpages to different users. E.g., I see
that the URLs generated by Horde have changed, but only for some users,
not for others.
How can we change it back? And what may have caused this? How can we
prevent these apparently unsolicited changes in the future?
Thanks in advance!
DrO