File uploading issue (client proxy)

I trying to help my Nextcloud user. We try to use a publicly available NextCloud server.
Login, logout, file download works fine. But upload is not possible. After selecting file to upload and click OK we see “a few second” bar and after timeout we see “an unknown error” message.
I saw so very small file (about 1kB) are possible to upload without above error.

I have a proxy server in my LAN. I try to upload files without a proxy and this works fine.
Is Nextcloud webapp compatible with a proxy server in user side?
I have no others issues with my proxy server.
I made a trial and login to via my proxy - and uploading files is posible.
How can I resolve this?