Tons of advertisements on log: Undefined array key XXXX at /var/www/html/lib/private/Share20/ShareHelper.php

As stated in the support template, we need a full relevant log entry not the excerpt:

Please provide the log entries from your Nextcloud log that are generated during the time of problem (via the Copy raw option from Administration settings->Logging screen or from your nextcloud.log located in your data directory). Feel free to use a pastebin/gist service if necessary.