Try to re-upload oc-includes folder manually. Maybe there was temp issue and 1 file did not updated.
I have re-uplod the oc-includes folder manually, but the issue isn't fixed.
I can narrow down the problem. The problem does not exist with the Sigma theme.
I then deactivated all plugins as a test with the Zeta theme. And then the problem did not occur.
So I activated the plugins step by step.
The problem occurs when I activate the Favorite Items plugin (version 2.5.3). I have tested it several times, it is due to the Favorite Items plugin, as soon as the Favorite Items plugin is activated and a user logs in, then 404 is shown.
I also re-uploaded the Favorite Items plugin, but this did not solve the problem.