At my radius server I have configured it to work with PEAP/EAP-GTC, each user has a user name and password. On other supplicants like NetworkManager, I'm able to successfuly authenticate. When using Xsupplicant, I try two different settings:
EAP-PEAP/EAP-GTC
-> This won't work because xSupplicant doesn't allow me to set a user password, only username. I don't know why this happens.
EAP-FAST/EAP-GTC
-> Even though I can specify the username and password, this authentication type isn't supported by the radius server because I have configured there EAP-PEAP/EAP-GTC.
Concluding, it seems that the implementation of EAP-PEAP/EAP-GTC isn't correct at XSupplicant, mainly because I can't specify username and password. On other supplicants it is allowed and the authentication works as expected.