Issue sending mails no longer possible SASL error

[TobiasP]

since the plesk update to 18.0.34 yesterday a lot of clients can't send mails anymore, the syslog shows:

Mar 12 10:37:59 carbon postfix/smtpd[21268]: warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
Mar 12 10:37:59 carbon postfix/smtpd[21268]: warning: unknown[xxx]: SASL LOGIN authentication failed: generic failure
and also:
Mar 12 10:34:59 carbon postfix/smtpd[20459]: warning: SASL authentication failure: no secret in database
Mar 12 10:34:59 carbon postfix/smtpd[20459]: warning: unknown[xxx]: SASL CRAM-MD5 authentication failed: authentication failure

content of /etc/postfix/sasl/smtpd.conf:
pwcheck_method: auxprop saslauthd
auxprop_plugin: plesk
saslauthd_path: /private/plesk_saslauthd
mech_list: CRAM-MD5 PLAIN LOGIN
sql_engine: intentionally disabled
log_level: 4

on another plesk server of mine "service saslauthd status" outputs active but on this one it outputs:
Unit saslauthd.service could not be found.

the server is debian 9.13

I tried

plesk repair installation
plesk repair mail
/usr/local/psa/admin/sbin/mchk --with-spam

but problem still persists

EDIT:

I removed
compatibility_level = 2
from postfix main.cf and now it seems to be working again... but on the other servers where there was no issue this is still present...

are there any side effects to removing (and therefore setting it to =0)? is there anything else I should check? can this setting come back? is there anything else I should do to prevent this in the future?
kind regards