8206361 created the issue

In general I would assume that this is a web server related topic, but I tested it with the official Nextcloud client, RaiDrive and via browser and the upload went through. Only when using Mountain Duck with online mode and smart sync mode the upload failed.

It only happens when the file is larger than 900MB or the upload lasts longer than a specific amount of time.

When the smart index mode is used Mountain Duck will repeatedly try to upload the file, which causes a constant use of bandwidth.

Tested it with Mountain Duck 4.3 where the file after upload is 0 byte and with the latest version 4.6 where the file is gone after upload.

The Nextcloud Log tells:

Error: Cannot modify header information - headers already sent by (output started at /home/vhosts/host.com/httpdocs/3rdparty/sabre/http/lib/Sapi.php:132) at /home/vhosts/host.com/httpdocs/apps/dav/lib/Connector/Sabre/File.php691

Like I said, other WebDav clients do not cause this error.