[CLIENT] Client will not open after pressing "launch" unless repeatedly ended in task manager

Having a unique issue that happens every time without fail: after pressing "launch" on the LoL patcher, the actual client will not open, ever. Upon retrying to open the patcher, a prompt claims that an instance of LoL is already running and must be closed. The patcher opens, I press "launch" again, and nothing happens. Again.

I've found that I have to open task manager and end all League related processes, and after three or four times of reopening the patcher and ending processes in the task manager, it will finally happen.

I have also uninstalled/reinstalled League on this computer with no success.

I would say this started happening around the latest major client UI update (months ago), but I kept putting it off because I figured this might be a common issue and would eventually be fixed with patches. This is not the case.