Same problem here. I also tried the mirrors proposed by @jsulm, no success. I'm using private computer in home network (no corporate laptops and networks).
[0] Arguments: qt-online-installer-windows-x64-4.8.1.exe, search, --accept-licenses, --accept-obligations, --email, *****************, --pw, ******
[12] Operations sanity check succeeded.
[26] Using metadata cache from "C:/Users//AppData/Local/cache\qt-unified-windows-online"
[26] Found 0 cached items.
[27] QtAccount information:
[27] Login completed: false
[27] Email: "***************"
[28] Total number of licenses: 0
[28] Account type: 3
[526] Warning: No QtAccount credentials found. Please login via the maintenance tool of the SDK.
[1235] Warning: Network error: [ QNetworkReply::InternalServerError ] "An internal server error occurred"
[1236] Warning: An internal server error occurred
[1236] Warning: An internal server error occurred
[1236] Warning: No valid license found.
It doesn't matter if I start it with "search" or "install" argument
If I put wrong password, the error is "Authentication failed".