This indicates that tmpfs is possibly full. Take a look here:
Can you open a terminal with Ctrl + ALt + T? If so, enter this:
df -h | tb
and this:
du -h /run/user/1000 | sort -h | tb
and post the two generated links here.
If not, then log out of the existing session (so that you are on the SDDM screen) and open a vt with for example Ctrl + ALt + F3, enter your user and your password and do what I described above.