I fixed this - needed to extend the domain storage quota.  For some reason the new user was defaulting to using all the remaining quota and then consequently failing.  this doesnt make sense.

So I updated to 4.2 again and tried to access users in one of the domains which gives the error.  Same error.  The error entry in syslog is this:

The last error line relates to this line:
File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/ireddate.py", line 198, in utc_to_timezone
#012    return ft.strftime(format)
#012ValueError: year=1000 is before 1900; the datetime strftime() methods require year >= 1900

Mar 24 18:57:38 ms iredadmin: Traceback (most recent call last):
#012  File "/usr/lib/python2.7/dist-packages/web/application.py", line 257, in process
#012    return self.handle()
#012  File "/usr/lib/python2.7/dist-packages/web/application.py", line 248, in handle
#012    return self._delegate(fn, self.fvars, args)
#012  File "/usr/lib/python2.7/dist-packages/web/application.py", line 488, in _delegate
#012    return handle_class(cls)
#012  File "/usr/lib/python2.7/dist-packages/web/application.py", line 466, in handle_class
#012    return tocall(*args)
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/sqllib/decorators.py", line 33, in proxyfunc
#012    return func(*args, **kw)
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/controllers/sql/user.py", line 144, in GET
#012    msg=form.get('msg', None))
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/iredbase.py", line 191, in render_template
#012    return jinja_env.get_template(template_name).render(context)
#012  File "/usr/lib/python2.7/dist-packages/jinja2/environment.py", line 1008, in render
#012    return self.environment.handle_exception(exc_info, True)
#012  File "/usr/lib/python2.7/dist-packages/jinja2/environment.py", line 780, in handle_exception
#012    reraise(exc_type, exc_value, tb)
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/../templates/default/sql/user/list.html", line 23, in top-level template code
#012    {% from "macros/msg_handlers.html" import user_msg_handler with context %}
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/../templates/default/layout.html", line 201, in top-level template code
#012    {% block main %}{% endblock %}
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/../templates/default/sql/user/list.html", line 157, in block "main"
#012    <i class="fa fa-cog fa-lg fr-space" title="{{ _('Edit account profile') }}{% if r.passwordlastchange %} ({{ _('Password last change:') }} {{ r.passwordlastchange | utc_to_timezone(timezone=session.get('timezone')) }}){% endif %}"></i>
#012  File "/opt/www/iRedAdmin-Pro-SQL-4.2/libs/ireddate.py", line 198, in utc_to_timezone
#012    return ft.strftime(format)
#012ValueError: year=1000 is before 1900; the datetime strftime() methods require year >= 1900
Mar 24 18:57:38 ms iredadmin:
Mar 24 18:57:38 ms iredadmin: [82.129.66.113] GET /iredadmin/users/bario.com 500 211 "https://ms.bario.com/iredadmin/profile/ … /bario.com"

Which iRedAdmin-Pro web page did you access to trigger this error?
Seems there's a date with year 1000, and Python requires it to be no earlier than 1900.

When uploading video files - the message disappears and vanishes?

Well, since your data default for datetime is 1000-01-01 00:00:00 and this is what caused the error.....If no-one had ever changed their password this is what it was - but why your latest version caused a problem only you will know I guess.

`passwordlastchange` datetime NOT NULL DEFAULT '1000-01-01 00:00:00',

However, I exported and verified some accounts indeed had the default, so I modified the data in vmail database, mailbox table:

update mailbox set passwordlastchange = '1971-01-01 00:00:00' where passwordlastchange = '1000-01                            -01 00:00:00';

This changed some of the accounts.

Then reinstalled 4.2 and all seems fine now...
But now of course I need to upgrade to 4.3...