### Steps to reproduce

1. Upgrade OpenProject to version 15.2.0

2. Begin of the insertionLogin as an admin user, navigate to Admin > Users and permissions > User settings > Display format (selectEnd of the insertion Begin of the deletion Open a Work Packages create beforeEnd of the deletion the Begin of the insertionoption that includes only the _firstname- this will be shown as your own firstname_) > Click SaveEnd of the insertion Begin of the deletion updateEnd of the deletion

3. Begin of the insertionNavigate to a work package that has a User custom field - if you do not have one, create one (please see [docs here](https://www.openproject.org/docs/system-admin-guide/custom-fields/#custom-field-formats))

4. Attempt to assign a group as the value in the User custom field you created in (3) or toEnd of the insertion Add a comment and mention someone in the comment or try to change the assigne of the issue

Begin of the insertion5.End of the insertion Begin of the deletion 4.End of the deletion The error message `missing attribute 'lastname' for Group` is shown Begin of the insertionand "An internal error has occurred"End of the insertion


### What is the buggy behavior?

* It is not possible to add comments to a work package

* It is not possible to change details of the work package

* It is not possible to change the assignee of a work package


### What is the expected behavior?

1. Comments can be added

2. Details and assignees can be changed


### **Logs**

Using the openproject utility, I get the following entry, when trying to add a comment to a work package:
`Jan 28 13:10:30 openproject.intra.platomo.de openproject-web-1.service[274974]: I, [2025-01-28T13:10:30.513599 #274974]  INFO -- : [7d22c4b7-3753-4c28-92ba-9eac1c0442e7] method=POST path=/work_packages/6776/activities format=*/* controller=WorkPackages::ActivitiesTabController action=create status=500 allocations=9473 duration=16.03 view=0.03 db=6.59 user=9`

### Screenshots and other files

<br>

_<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/290240/content">_ Begin of the insertion

<br>

_<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/300381/content">_End of the insertion

### Environment information

**OpenProject installation type**

* Packaged installation

* Ubuntu 22.04

* Installed from `https://dl.packager.io/srv/deb/opf/openproject/stable/15/ubuntu 22.04 main`


**OpenProject version**

15.2.0

**Browser**

* [x] Chrome
* [ ] Firefox
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [ ] Windows
* [x] Mac OS X
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)

**Language**

German