imap error since upgrade to php 7.1

Today I upgraded my php version to 7.1, and while the rest of wordpress seems to work fine, postie gives me grievances when fetching the mail from the imap server.
With php 5.6 I had no troubles…

The debug log outputs this:

Starting mail fetch
WordPress datetime: 2019-07-13 21:19:14
doing postie_session_start
wp_content_path: /home/www/swordpress/wp-content
OS: Linux s58.goserver.host 3.2.0-6-amd64 #1 SMP Debian 3.2.102-1 x86_64
PHP version: 7.1.2
PHP error_log:
PHP log_errors: Off
PHP get_temp_dir: /tmp/
PHP disable_functions:
PHP cURL version: 7.26.0
MySQL Version: 5.5.5
WordPress Version: 5.2.2
WordPress Multisite
network_home_url(): https://nomorenosiness.aberdoch.de/
WP_TEMP_DIR: (none)
WP_HOME: (none)
home_url(): https://markus-spring.info
WP_SITEURL: (none)
site_url(): https://markus-spring.info
WP_DEBUG: Off
WP_DEBUG_DISPLAY: null
WP_DEBUG_LOG: Off
WP_MAX_MEMORY_LIMIT: 256M (default)
WP_MEMORY_LIMIT: 96M
…
memory at start of email processing: 44666808
getemails: Connecting to s58.goserver.host:143 (imap)
getemails: procol: imap
pConnection: mailbox: INBOX
Connecting via Socket
Socket: s58.goserver.host:143
Socket error: 0 – (Resource id #813)
Socket: IMAP
socket write: (login) LOGIN/APOP/PASS
socket read (2) :* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE STARTTLS AUTH=PLAIN] Dovecot ready.
socket write: A0002 SELECT “INBOX”
socket read (9) :* FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
getemails: listing messages
socket write: A0003 STATUS “INBOX” (MESSAGES)
socket read (2) :* STATUS “INBOX” (MESSAGES 1)
socket write: A0004 FETCH 1:* (UID INTERNALDATE RFC822.SIZE)
socket read (2) :* 1 FETCH (UID 404 INTERNALDATE “13-Jul-2019 21:17:27 +0200” RFC822.SIZE 1421590)
Array
(
[404] => Array
(
[uid] => 404
[received] => 13 Jul 2019 21:17:27 +0200
[size] => 1421590
)
)
getemails: fetch 404
socket write: A0005 UID FETCH 404 (BODY[])
socket read (19235) :* 1 FETCH (UID 404 FLAGS (\Seen \Recent) BODY[] {1421590}
handlePart: type multipart/mixed

A post gets created, but empty and without the attachment I had sent with it.

Maybe I’ve overlooked something obvious, but I can’t see any reason for that behaviour.

Any help would be greatly appreciated

The page I need help with: [log in to see the link]