Mail delivery fails after install with error 10026: Connection refused

Mail delivery on a freshly installed Ubuntu 20.04 system with Rspamd fails with the error:

connect to 127.0.0.1[127.0.0.1]:10026: Connection refused

Log excerpt:

Sep  7 07:06:34 server1 postfix/smtpd[373776]: NOQUEUE: filter: RCPT from localhost[127.0.0.1]: test@test.int: Sender address triggers FILTER lmtp:[127.0.0.1]:10026; from=test@test100.int to=test@test100.int proto=ESMTP helo=<localhost>
Sep  7 07:06:34 server1 postfix/smtpd[373776]: 470B28261C: client=localhost[127.0.0.1], sasl_method=LOGIN, sasl_username=test@test.int
Sep  7 07:06:34 server1 postfix/lmtp[373882]: connect to 127.0.0.1[127.0.0.1]:10026: Connection refused

Workaround:

Login to ISPConfig, go to System > Server config > mail, switch spamfilter from rspamd to amavis, click save, switch back to rspamd and click save.

I'm not sure though if it's an error in the auto-installer or ISPConfig itself.