Issue Milter SPF check times out on certain domains

[Cappa]

Server operating system version

CentOS 7

Plesk version and microupdate number

Obsidian 18.0.45

Obsidian 18.0.45

Currently incoming mails from certain domains got stuck in the SPF milter check.
This happened suddenly, we are running this Plesk mail setup for over 3 years now.

The maillog shows this:

example 1 (google):
postfix/cleanup[22036]: warning: milter inet:127.0.0.1:12768: can't read SMFIC_BODYEOB reply packet header: Connection timed out
postfix/cleanup[22036]: 2535D40926: milter-reject: END-OF-MESSAGE from mail-lf1-f50.google.com[209.85.167.50]: 4.7.1 Service unavailable - try again later; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail-lf1-f50.google.com>

example 2 (microsoft):
postfix/cleanup[23472]: warning: milter inet:127.0.0.1:12768: can't read SMFIC_BODYEOB reply packet header: Connection timed out
postfix/cleanup[23472]: 577F7407F0: milter-reject: END-OF-MESSAGE from mail-dbaeur03on2105.outbound.protection.outlook.com[40.107.104.105]: 4.7.1 Service unavailable - try again later; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<EUR03-DBA-obe.outbound.protection.outlook.com>

Disabling SPF in Plesk on incoming mails solves the problem for now, these mails end up in the mailbox after disabling.
Increasing the timeouts is not a possibility / real solution.

Anyone has seen this or can think of a possible solution?