maybe this way you can find the culprit:
$ lsof -U +c 15 | cut -f1 -d' ' | sort |
uniq -c | sort -rn | head -3
And the winner is: thumbnail.so
I found the way to reproduce the problem.
In Dolphin, select a file. Right click and select delete. Press Ok in confirmation dialog.
When pressing Ok a new "thumbnail.so [kdeinit5] thumbnail" process appears in state S (interruptible sleep (waiting for an event to complete) according to ps) and a new "Qt Selection Owner for thumbnail.so" entry in xrestop. When entries reach 256, X stops to open new windows.
Fortunately a simple killall -9 thumbnail.so solves the problem.