Hello @Pandananana,
You’ve stumbled upon a known limitation of many modern Linux distributions. They simply do not handle multiple graphical sessions for the same account very well. This also holds true for some applications, like browsers and mail clients.
While ThinLinc technically allows for multiple connections for one user, the limitation comes at play with the Linux desktop and DBUS/SystemD subsystems.
Please see Why does starting multiple ThinLinc sessions as the same user sometimes fail? for more information.
Regards,
Martin