PoE changes windowed fullscreen to fullscreen every day...

Every day I have to switch back to windowed fullscreen (I normally use my 2nd monitor and fullscreen minimizes the game).
When I change it, it will stay windowed for the day but every day it changes back to normal fullscreen...

Sounds like a permissions issue on your system.

It’d help to know which OS you use, which client you installed, and the file path of the game client.

It's Windows 10, Steam client and the normal steam file path. I didn't change anything with steam or the game in years.

Currently loading the new geforce update, maybe that's the case.

Is OneDrive configured to backup the Documents directory on your PC?

That would interfere with Path of Exile's ability to make permanent changes to the below file;

%USERPROFILE%\Documents\My Games\Path of Exile\production_Config.ini

(You should be able to copy & paste that into File Explorer's address bar.)

I don't use OneDrive, no. But the geforce update seems to work. After extensive testing yesterday and starting it now for the first time of the new day it didn't change.

Thanks for your help, even when it was my fault/stupidity ;)