Hi Yannick,
The issue arises from a schema conflict between Joomla 5's updated schema and the T3 framework, which already implements a similar schema.
To resolve this, please follow one of these steps:
Option 1: Replace the file
Back up the file: root/plugins/system/t3/base-bs3/html/mod_breadcrumbs/default.php
Replace it with the attached file (unzip first):
Option 2: Add to template override folder
Copy the unzipped file to: root/templates/your_default_template/html/mod_breadcrumbs/
Create the folder path if it doesn’t exist.
Let me know if you need further assistance!
Best regards
Regards