After renaming, the folder has disappeared

On Nextcloud 16.0.5 we are experiencing an odd behaviour:

User A has created a folder and shared it with other users (among which we have user B);
User B has renamed the folder;
User B can see the renamed folder while user A cannot see the folder any more it just vanished from web interface…
User B still sees the folder as owned by user A.

All of this happened on web interface, not through the client.

How can user A regain access to the folder he owns?

Logs show no errors , server is Ubuntu 18.04.03 LTS with LAMP , PHP is 7.2.19, Mysql 5.7.27, Apache 2.4.19.
The sever is behind HAproxy reverse 1.7.x with SSL offloading.