KDE always gives you enough rope to hang yourself. Like, set the transparency of all windows to 100% and wonder why the system is fucked, or whatever haha.
Working blind, and from memory (I didn't check my system): depending on your system, there will be a kwin config file in .local or .config or .kde or similar in your home directory. Assuming you have console access, df -h | grep kwin will probably find it for you. Take a peak in the file first to make sure it's reasonable that this is the right file to nuke. Rename it something like kwinrc-backup and restart KDE.