The weekly backup job on one of the sites I manage suddenly started showing a warning during weekly backups created after 3\/4\/26. The log is below, showing the warning, which appeared on both the 3\/11\/26 and 3\/18\/26 backups. <\/p>\n\n\n\n
No changes were made to the job in this timeframe. The only things I changed between 3\/4\/26, when the job last ran without the warning, and 3\/11\/26, when it first ran with the warning, were:<\/p>\n\n\n\n
3\/5\/26 – BackWPup 5.6.5 > 5.6.6<\/p>\n\n\n\n
3\/9\/26 – Burst Statistics 3.2.0 > 3.2.1 (I applied a subsequent update to 3.2.3 on 3\/12\/26, warnings recurred on the 3\/18\/26 backup)<\/p>\n\n\n\n
3\/10\/26 – WordPress core – 6.9.1 > 6.9.3<\/p>\n\n\n\n
Both the zip and zliib PHP extensions are enabled (PHP 8.3). <\/p>\n\n\n\n
[INFO] BackWPup 5.6.6; A project of WP Media
[INFO] WordPress 6.9.4 on
[INFO] Log Level: Normal
[INFO] BackWPup job: Weekly Full Site Backup
[INFO] Logfile is: backwpup_log_2026-03-18_01-36-37.html
[INFO] Backup file is: 2026-03-18_01-36_backwpup_DBDUMP-FILE-WPPLUGIN.zip
[18-Mar-2026 01:36:37] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:36:37] 1. Try to backup database \u2026
[18-Mar-2026 01:36:37] Connected to database on localhost
[18-Mar-2026 01:36:39] Added database dump with 32.45 MB to backup file list
[18-Mar-2026 01:36:39] Database backup done!
[18-Mar-2026 01:36:39] 1. Trying to make a list of folders to back up \u2026
[18-Mar-2026 01:36:40] Added \"wp-config.php\" to backup file list
[18-Mar-2026 01:36:40] 1391 folders to backup.
[18-Mar-2026 01:36:41] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:36:41] 1. Trying to generate a file with installed plugin names \u2026
[18-Mar-2026 01:36:41] Added plugin list file \"_.pluginlist.2026-03-18.txt\" with 2.30 KB to backup file list.
[18-Mar-2026 01:36:41] 1. Trying to generate a manifest file \u2026
[18-Mar-2026 01:36:41] Added manifest.json file with 2.85 KB to backup file list.
[18-Mar-2026 01:36:41] 1. Trying to create backup archive \u2026
[18-Mar-2026 01:36:41] Compressing files as ZipArchive. Please be patient, this may take a moment.
[18-Mar-2026 01:37:08] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:37:44] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:38:03] Backup archive created.
[18-Mar-2026 01:38:03] Archive size is 717.35 MB.
[18-Mar-2026 01:38:03] 9213 Files with 896.06 MB in Archive.
[18-Mar-2026 01:38:04] WARNING: ini_set(): Cannot change zlib.output_compression - headers already sent
[18-Mar-2026 01:38:04] 1 old log deleted
[18-Mar-2026 01:38:04] WARNING: Job finished with warnings in 87 seconds. Please resolve them for correct execution.<\/code><\/pre>\n\n\n\nAny thoughts about what the problem might be? I manage many sites (in separate accounts) on this host, and am not seeing this warning anywhere else. <\/p>\n","protected":false},"template":"","class_list":["post-18856015","topic","type-topic","status-publish","hentry","topic-tag-headers-already-sent","topic-tag-log","topic-tag-warning","topic-tag-zlib-output_compression"],"jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/topic\/18856015","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/topic"}],"about":[{"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":1,"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/topic\/18856015\/revisions"}],"predecessor-version":[{"id":18856016,"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/topic\/18856015\/revisions\/18856016"}],"wp:attachment":[{"href":"https:\/\/wordpress.org\/support\/wp-json\/wp\/v2\/media?parent=18856015"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}