LDAP group not recognized for new users in Pydio 8

hi guys, I’m running Pydio 8 community edition, for a charity association. We configured it on Debian 8, running with MySQL and OpenLDAP.

It’s been running for several years now, but we just recently realized that after we upgrade pydio 7 to pydio 8 something in the LDAP config is not working anymore.
Users are working fine, all groups are also correctly visible, it’s just that the most recent users (the ones created in the LDAP after upgrade) do not get mapped to any group (despite they are indeed mapped to the right group in the LDAP).
Older users do not have this problem, they’re still recognized as part of their group.
I’ve disabled caching master users in the slave configuration, but this didn’t have any effect.
Could you give me some hint on how I can debug this?

thanks,
Silvio