ispconfig3_account plugin - Soap Error: Call to a member function queryOneRecord() on false

I'm trying to install the plugin package for changing passwords, forwarding, etc., for Roundcube.
I'm doing everything correctly, just like in other installations I have.
But on this new server I'm encountering this error:

Soap Error: Call to a member function queryOneRecord() on false

I checked the logs and didn't find anything relevant.
The remote user exists, everything is correct, but it's not working. Any ideas?