Why is the menubar icon red?

I have a nextcloud server setup using caddy and php fpm syncing a OpenZFS volume on my Mac. It appears that the sync has worked save for 40 files, which have very complex names which I think is what’s tripping up nextcloud. It reports errors like

Wrong HTTP code returned by server. Expected 201, but received "0 status code 000".
The server did not acknowledge the last chunk. (No e-tag was present)

An example if it lets me print it here is
iPhoto Library/Thumbnails/2014/04/21/20140421-000207/%2fugJZrRNavNL8t74F7yg

The menubar icon of nextcloud is red. Is it red because there are files it can’t sync? How can I teach it to exclude files that can’t be synced?

Also, could it be the cache system you’re using.
Are you using redis?