Hi,
sorry i missed your mesaage, i am checking the site right now but i am not sure what might be causing this problem.
The only thing i noticed is that the wfvt_304361968 cookie changes with every request i do not know if that should be the case, either way this cookie is generated by WordFence plugin as far as i know so you can try disabling it and see if the upload will work.
If you have other plugins for managing cookies you can try disabling them as when i check the cookies sent/received in WordPress headers there is quite a lot of default cookies missing which might be causing a problem, because the session is verified with nonces (https://codex.www.ads-software.com/WordPress_Nonces) which are saved in the database and assigned to a user using cookies.