Eight fixes, led by one that could make the whole server hang for everyone.
docker pull ghcr.io/new-usemame/calibre-web-nextgen:v4.1.15
Or :latest for the newest stable, :dev for the canary channel.
Fixed
"Change cover" made the whole server unreachable until it finished. Opening the cover picker on one book froze every other page for everyone using the server — up to about 12 seconds, however long the slowest metadata source took to answer. The "Search metadata" button on the edit page did the same. On a test library, a book page that normally answers in 30ms took 11.4 seconds while a cover search ran; it now answers in well under a quarter second, and the cover search itself is no slower. Thanks to @darkmatterpelican for reporting it.
Books with two or more authors were hard to read in the new interface. Authors were separated with a comma, but an author's name can contain a comma itself ("Dumas, Alexandre"), so two authors came out as "Dumas, Alexandre, Maquet, Auguste" with no way to tell where one person ended and the next began. Authors are now separated with "&", the same way the classic interface, Calibre, and the new UI's own edit box have always done it. Tags, languages and publishers are unaffected. Thanks to @chloeroform for the report.
Setting a default library view turned your library into the search page. After saving a default view, the library home showed the "Advanced search" form pinned above the books, the page was retitled, and the library heading and Discover strip disappeared. Your library now stays your library — it shows the books your default view selects, with a note saying so and a "Show all books" link. Reported by @chloeroform.
New accounts ignored the default theme you picked in Admin. Whichever theme an admin chose, some new accounts still started on Dark: people who registered themselves through the new UI got it wrong on servers upgraded from an older build, and accounts created by OAuth, LDAP import, or an external/proxy login always got Dark. Admin-created accounts were always fine, so one server could produce two different results. All seven ways an account can be created now seed the theme you configured.
Automatic duplicate resolution never ran if you set a cooldown. Turning on the cooldown stopped automatic duplicate resolution entirely, and the log reported a wait counting up rather than down. Two causes: the cooldown you typed was discarded and replaced with one minute, and the clock comparison mixed local time with UTC. If your server runs east of UTC the opposite happened — the cooldown was ignored and resolution ran on every scan. Your existing resolution history stays intact. Thanks to @jdbway, who diagnosed both causes and pinpointed the exact lines.
KOReader stopped seeing new versions of the sync plugin. Updating the plugin from inside KOReader kept offering the version from 13 July, even though three plugin fixes had shipped since: highlights and notes syncing into your library, highlight deletions syncing to the server, and a guard that stops a device deleting highlights it never had. The plugin releases those tools read had quietly stopped being published. The current plugin is published now, and publishing it is no longer a manual step that can be missed. If you install the plugin by downloading it from your own server's KOReader page, nothing changed for you. Thanks to @KucharczykL for spotting it and to @filiporlo for the original work.
Startup no longer sets permissions on your Calibre library twice. Every container start walked the whole library once from a hardcoded list and again from dirs.json, and re-walked a folder inside /config that had already been covered. Each folder is now visited once, which shortens startup on large libraries. Thanks to @chloeroform for spotting it in the startup log and measuring it.
Russian screen readers announced the reader's progress as "Прочитано: 45% r" — a stray letter left over from the English "read". It only ever reached people using a screen reader, since the text is spoken rather than drawn on screen. Brazilian Portuguese was already correct.
Notes
Patch releases are safe to auto-update: no config, database, or API changes.
Something still broken? Open an issue at https://github.com/new-usemame/Calibre-Web-NextGen/issues — the tracker is watched.