Sounds as if you're using the nVidia drivers and not nouveau.
Correct?
yes
If so, where/how did you install the nVidia drivers?
do remember, I had them with F25.
Nevertheless, I have now removed all \*nvidia\* packages and I have
now 2 issues:
1. screen resolution is wrong in sddm
2. I still cannot open a plasma session with the following error in dmesg:
[ 113.938502] sddm-helper[1269]: segfault at 0 ip 00007f3f74b96b6e sp
00007ffca1ba7300 error 4 in libc-2.26.so[7f3f74b1f000+1da000]
F