Adding new Pages to Homepage results in 404

I am adding new Pages to my Page:

And after hitting save, Grav redirects me to:

But you can see that the Page already exists in the Menu-Bar. And here you can see it in Admin-Plugin:

I can then go back via Browser and save the Page again. The Save Button is then not clickable but "looks" like it can be clicked (not disabled).

When I refresh the Page (F5) the page reloads and is empty but existing then:

I can now correctly edit it and save it. I dont have any Error-Logs to show as no real Error seem to occur.

It was not like this all the Time. I had the Plugin "WebP" installed (https://github.com/greenrivers/grav-plugin-webp) and this somewhat broke my Site with an Error:

grav.CRITICAL: Call to undefined method GuzzleHttp\Utils::chooseHandler()

I have removed the Plugin and the described stuff from above started to happen.

Cache (method: all) has already been cleared.
Admin Panel Version: v1.10.48
Grav Version: v1.7.48

What could be the Issue?

How come you add the page to / (root), but URL is to /pages/test? Where soes the /pages part come from?

Its the Pages-Part of the Admin-Plugin where you can Edit the Pages you have.

And this View with the Error should be the Edit-Window but I think Grav redirects to a different Place then the Edit-Window.

So it sounds like this redirect losses the /admin part somewhere. Try disabling plugins one by one or just fully reinstall Grav and restore user folder after

I did a full reinstall of Grav and restored user-Folder.

Issue persists, how can I check the User Folder for Issues? Maybe the Theme got changed.

Did you try disabling plugins one by one to see if that fixes it?

Huh thought it was something about the State of Grav itself but I went with your approach and disabled Plugins until I found out that disabling "Flex Objects" helped and everything seem to work now.

Page can be created and saved as usual. No error or 404 anymore.

What is the next Step? No changes on Plugin (Updates) in near past.

Do you think a fresh install of the plugin could help? Can I break something by deleting an re-install?

@Snickbrack:
Do you think a fresh install of the plugin could help? Can I break something by deleting an re-install?

Strange. Flex Objects come by default with Grav. You can try deleting it and re-installing. You already must have a backup, so should be no harm done

Okay its getting more strange now.

I have deleted the Plugin and copied it from fresh skeleton, updated it, enabled it again.

Issue persists.

I dont know where this is coming from.

Does it work without updating? Did you try older versions?