### Steps to reproduce

1. Go to Go to Administration > Authentication > SAML providers

2. Set up a new provider

3. Begin of the insertionFill something for the Mapping > UID attribute

4. Save

5. Edit again, clear that mapping

6. End of the insertion Create a new user account on the SAML side
alternatively

Begin of the insertion7.End of the insertion Begin of the deletion 4.End of the deletion Create a new account on the SAML side

Begin of the insertion8.End of the insertion Begin of the deletion 5.End of the deletion Go to OP

Begin of the insertion9.End of the insertion Begin of the deletion 6.End of the deletion Click on SAML and follow the login steps


### What is the buggy behavior?

* Login fails

* Error message is not helpful (which attribute?)


### What is the expected behavior?

1. Login is successful


### Important note

The attributes are set automatically and i didn't change them so there is no reason why they should throw an error

I set up the SAML provider twice and it still didn't work

I used the same provider to test OIDC and it worked successfully

### Screenshots and other files

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/226608/content">

### Environment information

**OpenProject version**

Core build

[cb3a6297e8013de85f7142e7d44674f69a130e6a](https://github.com/opf/openproject/commits/cb3a6297e8013de85f7142e7d44674f69a130e6a)

Product version

[be995c43815204d2c47bc347915ead8fc546d134](https://github.com/opf/saas-openproject/commits/be995c43815204d2c47bc347915ead8fc546d134)

Core version

OpenProject 15.0.0

**Browser**

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

**Operating System**

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