remote connect doesn't work

Hi @all.

After updating from 8.1.1 to 9.1.1, the RemoteConnectPlugin no longer works. The port is open, in the configuration we used both the DN and the ObjectID of the AD group. We always get the error

The underlying connection was closed: An unexpected error occurred on a send.
Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
An existing connection was forcibly closed by the remote host

Port 2880 is open, there is no entry in any log that helps us.

I know there was a fix for 8.1.5, but we skipped that version.

Any Ideas?

Best
 Gregor