@bruno Your problem seems either graphical because you can log in with startx or startplasma-x11 or a problem with sddm. Be sure sddm.service is enabled with sudo systemctl status sddm.service and if it is not run sudo systemctl enable sddm.service. We have seen users needing to enable this for some reason. (OK, honestly I forgot the reason…) If not that then for developers to resolve the problem you are seeing the logs explained below would help.
For any user: For people having unsuccessful booting we need logs of that. To get logs of a previous boot:
journalctl -b -x | tee journal-unsuccessful-boot.txt
journalctl -k -b -x | tee dmesg-unsuccessful-boot.txt
where -x is < boot_number > (How many reboots ago 0, -1, -2, etc.) 0 being current boot. -k provides dmesg output, so there are two different commands above with differing output. Also the commands create the .txt files you can post here in this forum or in your bug report. You can list previous boots with:
journalctl --list-boots
These are listed with dates/times so if you know the date/time of an unsuccessful boot you can find it in this list.
Hope this helps.