From the backtrace, it looks like the error is due to the participantlistemaildisplay (https://github.com/voiply/com.servnettech.participantlistemaildisplay) extension enabled on your site.
To fix -
Try disabling this ext from civicrm/admin/extensions?reset=1.
Create an issue on the extension repo - https://github.com/voiply/com.servnettech....
this can be achieved using an event price set. If you create a price set you can create a ticket option which takes up multiple "seats" at the event. You can create a 10 person ticket option and then set the price of that option.
Note however in that approach you cannot require that all 10 attendees submit separate registration details and you can only ...
Shaun, Welcome to CiviCRM SE,
You will need to
Add a custom group extending participant (CiviCRM >> Administer >> Customize Data and Screens >> Custom fields)
Add custom field of type select/radio/checkbox in a custom group created in #1.
Include this custom field in your profile which you are thinking to include for event.
Note: Custom field UI doesn't ...
Currently the string in question isn't properly translated into Dutch.
CiviCRM's translation project(s) live on Transifex. There are two "teams" (to use Transifex's nomenclature!) for translation.
CiviCRM/CiviCRM - Core Translations
CiviCRM/CiviCRM_Extensions - Extension Translations
Within each "team" the translations are broken down into "...
As a quick fix, you can use the word replacement feature of CiviCRM, which is located in the admin section. Just note that the character '&' is encoded as a html special character. Therefore the original English string you want to replace is Terms & Conditions. Hope this helps.
if you carry out a contact search (selecting the group your interested in) and select all of the results, one of the actions you see in the actions menu will be to add them to an event (Register Participants to an event). Have you tried that approach?
Once registered you'll be able to carry out all the tracking/reporting you've highlighted.
At administer - system settings - directories, there's one called custom php path that contains a folder path. Go look in that path in the filesystem and you probably have a customized version of CRM/Core/BAO/UFGroup.php. It will need to be updated with some recent changes that were made to the real CRM/Core/BAO/UFGroup.php, in particular the addition of the ...