I faced problem like below when I upload file.
An unknown error has occurred
Conclusion is HTTP 413 Request Entity Too Large happened in my environment.
I expanded upload limit size to 128MB.
First is https-portal which acts as reverse proxy.
You only need to insert this line.
CUSTOM_NGINX_YASUFUMI_YOKOYAMA_GQ_CONFIG_BLOCK: 'client_max_body_size 128M;'
Next is configuraiton of Nginx.
You need to add this line to server block.
After editing them, you need to restart each Nginx!
How was it?
You can do it in 5 mins!