Another post that reports upload errors:
This fixed the issue for me: login to your Seafile web UI, click on your avatar, got ot “System Admin” > Settings > FILE_SERVER_ROOT > and here you should put your seafile URL or IP address like this:
“hxxp://{YOUR_SERVER_URL}/seafhttp”
For exmple, my server URL is “seafile.local” so I put
hxxp://seafile.local/seafhttp
You can also put it in ‘SEAFILE_SERVER_HOSTNAME=’ in the docker-compose.yml file.
Keep in mind: if the URL is a domain name like (seafile.local), you should have a record for it in your DNS server.
I’m using Pi-Hole as my local DNS server and I’ve added a record for seafile.local pointing to my server’s IP and then put this domain in the system admin URL settings and I was immediately able to upload files.