I had a trouble with guacamole and RDP.

Looked forward to the certificate issue... but that wasn't the case.

activating the debug of guacd gave me the missing clue: krb5glue_get_init_creds (Cannot find KDC for realm "XX-XXXX" [-1765328230])

i googled for this issue and found https://github.com/FreeRDP/FreeRDP/issues/10138 where it pointed out the good real name to specify... so i finaly entered the FULL REAL NAME "XX-XXXX.xxxxxxx.xx in the domaine name in the rdp configuration. And, that worked.
The explanation: using administrator profile doesn't give any trouble with the short domain name because it appears he doesn't use kerberos auth... (silly but that's so. Will have to look forward this security issue, 'cause that means administrators use ntlm :-( )
But using another profile => using kerberos, and for that, we need the right full domain name to be mentionned in the rdp config.