### **Steps to reproduce**

1. Login as a user with permissions to create work packages in a project

2. Navigate to a project

3. Open a custom view within the project

4. Begin of the insertionConfigure a filter on a custom field of type date. Set that filter operator to "is empty".

5. End of the insertion Click the green button to create a new work package

Begin of the insertion6.End of the insertion Begin of the deletion 5.End of the deletion The input panel should open on the right-hand side

Begin of the insertion7.End of the insertion Begin of the deletion 6.End of the deletion Observe the result: only a blank (white) area is displayed instead of the form

Begin of the insertion8.End of the insertion Begin of the deletion 7.End of the deletion Check the browser console – a 500 error is shown for the following request:


`POST /api/v3/projects/abteilung-edv/work_packages/form`

Payload (excerpt):

`{  "_links": { Begin of the insertion End of the insertion Begin of the deletion  End of the deletion  "project": {"href":"/api/v3/projects/13"},    "type": {"href":"/api/v3/types/1"}  },  "assigneeOrGroup": {    "$source": {      "_links": {        "self": {"href":"/api/v3/users/3"}      }    },    "$loaded": true  },  "customField12": {    "href": null  } Begin of the insertion}`End of the insertion Begin of the deletion }` End of the deletion

### **What is the buggy behavior?**

* The panel for creating a new work package stays white (empty)

* A 500 internal server error is returned from the API

* The error response contains:


`{  "_type": "Error",  "errorIdentifier": "urn:openproject-org:api:v3:errors:InternalServerError",  "message": "Ein interner Fehler ist aufgetreten.invalid date" Begin of the insertion}`End of the insertion Begin of the deletion }` End of the deletion

### **What is the expected behavior?**

* The panel on the right should show the input form for a new work package

* The form should be prefilled based on the selected type and project

* No 500 error should occur, and the request should succeed

* The user should be able to complete and save the work package


### **Additional context**

* The project contains **two optional custom date fields**, one of which is used to **postpone tasks**

* In the custom view where the error occurs, a **filter is applied**: "Only show work packages where the "Postpone until" date field is **empty"**

* When trying to create a new work package from this view, the form fails to load with a 500 error

* Reloading the page (F5) allows the form to load correctly again


### **Logs**

* Browser console shows a 500 error for `POST /api/v3/projects/abteilung-edv/work_packages/form`

* Error message: `"Ein interner Fehler ist aufgetreten.invalid date"`

* No further log output was provided yet


### **Screenshots and other files**

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/405353/content">

### **Environment information**

**OpenProject installation type**
☑ Packaged installation (via APT on Linux)

**OpenProject version**
`15.4.2`

**Browser**
☑ Firefox `136.0.4`

**Operating System**
☑ Windows 10

**Language**

* Browser language: German

* OpenProject UI language: German (`Deutsch`)