Thank you for your reply. The project 'Pipeline4test' did just use to reproduce the problem. The real problem occurred when updating blt from 8.6.15 to 8.8.2 on an existing site. Does that answer your question?
This problem did not occur in 8.6.15. It seems like the #1200 that changed when updating to 8.7.x or 8.8.x seems to be affected, but I am not sure. However, errors came to occur in that place.