Am Mittwoch, den 01.04.2020, 15:41 +0300 schrieb Sami Ketola:
Hi,

"If personal user scripts also need to directly access external programs, the extensions need to be added to the sieve_extensions setting. "

from https://wiki2.dovecot.org/Pigeonhole/Sieve/Plugins/Extprograms

so is this a global sieve script or personal sieve script? I think it is personal script as it is triggered by imapsieve.

This is a global script. However, what now seems to have fixed it is adding 

sieve_plugins = sieve_imapsieve

and 

changing 

sieve_global_extensions = +vnd.dovecot.environment +vnd.dovecot.debug

to 

sieve_global_extensions = +vnd.dovecot.environment +vnd.dovecot.debug +vnd.dovecot.pipe

in conf.d/90-sieve.conf

Obviously, having these additions in the settings for the IMAP plugin wasn't sufficient.. Hm...

Cheers,

Johannes