I can confirm this.
This was working fine. Some update changed the situation.
Most users cannot create new events in their calendar. one however still can.
This is in roundcube. Event creation via caldav is not affected.

The date and time format under Settings in Roundcube can be identical between user that have this problem and user that don't. At least at this level that does not seem to make a difference.

We are also affected.

The curious thing: Only one user account is affected, regardless from which Browser. All other users can create events.

Maybe in relation to this problem: In an account of mine I can't create events earlier then 10 am. From the social view this is actually a good thing. But some people react a kind of bemused if I say sorry: My calendar isn't able to meet us that early.

But the difference to this bug here: The save button does an action at all and I get the error message "Invalid data". Creating a event after 10 pm and move it with the mouse works fine. But also here: only one account is affected.

For those useres affected, we could fix the problem by setting the roundcube standard date format from the German to the Englisch version. Then the save button works again. Nevertheless, those users, who did not have this problem, continue to use the German date format setting without a problem. All this on the same server and same domain.

In my case it was sufficiant to switch from German to English date format and back. Then it's possible to save events but the pop up window doen't close. It remains but without content.

In the meantime I got another issue - not with calendar but wit task plugin.

One user couln't save tasks anymore if he assigned them to another user. This could be solved by deleting the content of "preferences" column in roundcube table users for the specific account and change the skin afterwards. (Why ever these two steps where necessary).

This issue, mentioned above:

Maybe in relation to this problem: In an account of mine I can't create events earlier then 10 am. From the social view this is actually a good thing. But some people react a kind of bemused if I say sorry: My calendar isn't able to meet us that early.

could be solved also by resetting roundcube preferences.

It was reproducable - pasting again the former content of the preferences column again events before 10 am where not saveable.

Could you provide the faulty preferences?

Here are the preferences which cause the error. I have shortened the folder list for privacy reasons. But I think it doesn't cause the error.

a:38:{s:20:"kolab_active_folders";s:5610:"INBOX**Drafts**Sent**Spam**Trash**Trash/000_Support_tmp**Trash/Archives.2016**Trash/LUG-DD**Trash/ups4**Archive**Archive/2014**Archive/2016**Archives**Archives/2014**Archives/2017**Calendar**Configuration**Contacts**Contacts/Personal Contacts**Erledigt**Files**Freebusy**Journal**Notes";s:19:"calendar_first_hour";i:8;s:19:"calendar_work_start";i:9;s:17:"calendar_work_end";i:17;s:20:"calendar_show_weekno";i:0;s:23:"calendar_event_coloring";i:0;s:27:"calendar_default_alarm_type";s:0:"";s:29:"calendar_default_alarm_offset";s:4:"-15M";s:25:"calendar_default_calendar";s:11:"Q2FsZW5kYXI";s:26:"calendar_contact_birthdays";b:1;s:29:"calendar_birthday_adressbooks";a:1:{i:0;s:17:"kolab_addressbook";}s:29:"calendar_birthdays_alarm_type";s:0:"";s:31:"calendar_birthdays_alarm_offset";s:3:"-2D";s:19:"calendar_categories";a:5:{s:8:"Personal";s:6:"c0c0c0";s:4:"Work";s:6:"ff0000";s:6:"Family";s:6:"00ff00";s:7:"Holiday";s:6:"ff6600";s:5:"Calls";s:0:"";}s:8:"timezone";s:4:"auto";s:11:"date_format";s:5:"Y.m.d";s:11:"time_format";s:3:"G:i";s:16:"refresh_interval";i:600;s:4:"skin";s:13:"chameleon-dkn";s:10:"date_short";s:5:"D G:i";s:9:"date_long";s:9:"Y.m.d G:i";s:16:"collapsed_abooks";s:32:"&Q29udGFjdHM&&kolab_addressbook&";s:16:"message_sort_col";s:4:"date";s:18:"message_sort_order";s:4:"DESC";s:17:"collapsed_folders";s:302:"&Other%20Users%2Fflorian.rasch&&Other%20Users%2Fjan.kowalsky&&Trash&&Archive&&Archives&&Other%20Users%2Fexample.kontakt%2Fbuchhaltung&&Other%20Users%2Fexample.kontakt%2Fdomains&&Other%20Users%2Fexample.kontakt%2Fprojekte&&Other%20Users%2Fexample.kontakt%2Fselfregistrations&";s:11:"search_mods";a:4:{s:1:"*";a:2:{s:7:"subject";i:1;s:4:"from";i:1;}s:4:"Sent";a:2:{s:7:"subject";i:1;s:2:"to";i:1;}s:6:"Drafts";a:2:{s:7:"subject";i:1;s:2:"to";i:1;}s:32:"Shared Folders/shared/dknreceive";a:2:{s:7:"subject";i:1;s:4:"from";i:1;}}s:23:"addressbook_search_mods";a:5:{s:4:"name";i:1;s:9:"firstname";i:1;s:7:"surname";i:1;s:5:"email";i:1;s:1:"*";i:1;}s:14:"compose_extwin";i:0;s:10:"reply_mode";i:0;s:25:"compose_save_localstorage";i:1;s:12:"skip_deleted";b:0;s:17:"flag_for_deletion";b:0;s:15:"read_on_archive";b:0;s:15:"kolab_calendars";a:2:{s:44:"T3RoZXIgVXNlcnMvc2FuZHJvLm1lcmJkL0NhbGVuZGFy";a:1:{s:10:"showalarms";b:0;}s:9:"__bdays__";a:1:{s:6:"active";b:1;}}s:17:"acl_advanced_mode";b:1;s:14:"mail_read_time";i:-1;s:14:"kolab_uniqueid";s:32:"3652b1e917ed043602379fb579643d67";s:11:"client_hash";s:16:"wd7N6bWC1jIiLScN";}

By the way: the error mentioned above where one user couldn't save tasks as soon one person is assigned, still persists. And it wasn't reproducable to fix it by deleting the defaults. But I couldn't test it thoroughly because I'm not the account owner and I had to guide the customer by phone.

I'm unable to reproduce any of these issues.

this is the problem. It's not reproducable at all. Nearly all of our users can do all the stuff. Only very few encounter these problems. So maybe it's a caching issue - and the settings are not repsponsible but special settings together with some special circumstances in caching.