Can't connect to a certain secured wifi after upgrading to Win 10

Hey there, I need a laptop for my studies - that's why I looked for my HP 650 Laptop I used three years ago.

First thing I did was upgrading to Windows 10. I didn't have any problems doing that at home connected to my home wifi. When I'm at school there are two wifi networks: one secured wifi and one unsecured wifi. If you connect to the unsecured one a log in browser window pops up asking for a username and password. When connecting to the secured one the credentials are the exact same ones. BUT: I don't have any problems connecting to the unsecured one.

Problems occur when I try to connect to the secured (faster) one.
This morning I was able to use the secured one. After 2 or 3 hours I was kicked out and ever since I am not able to reconnect to the secured one. The latest drivers for the network adapter are installed. I also tried adding the network manually. I unchecked "Varify the server's identity by validating the certificate". Now I am asked to put in my username and password in a system window - but it doesn't go through and I am asked over and over again without any established connection.

Any ideas what to do?

You need to talk to the IT group at your school regarding your computer not able to connect to the network since you did the upgrade. Has nothing to do with Windows 10. It has to do with the security of the school network.