Solved Suddenly problems with Kunena Forum version 6.3.8

1781 is show the user profile which ist still working flawless.
I have added the PhocaGallery UCP recently. But even I have disabled it now, the errors are still there.
Any further idea?

Do you have a main menu on your site? I don't see that in your configuration report. Furthermore I can't see a Kunena menu in your second screenshot.
And I would delete the trashed menu-items.

 
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1. you must change these two menu items to menu item Alias.
Menu Issues  
2. delete all Kunena menu items that are in Trash.
.
 

3. the menu item 'Hauptforum' is linked to itself. Open it and select ‘Foren’ or ‘Aktuelle Themen’ as the destination, depending on where you want to end up.
Menu Tips
4. go to Kunena -> Tools in the backend and have the statistics counted and synchronise the users.

If that doesn't help, activate the Kunena debug mode (Kunena configuration -> General tab), you may get a more meaningful error message.
 

Thanks for your advice. Prior doing that, maybe you can help me on the following points:
- Why the error occured now, when the forum was running flawless before?
- When I run under Kunena Tools the Diagnosis, I receive the following error: topicMissingMessages Test Failed 2 Problems Delete Broken Elements
- Why should I also change the menu item of "Show Profile" from Joomla when this is still running without problems?
Would be great to get a reflection on this.
Thanks
Chris

Why the error occured now, when the forum was running flawless before?

Because you changed something. Or, but less likely, your site is hacked.
Just follow the advice @rich and I gave you.
Or restore a backup from when your site was without errors.

I have now done all changes as adviced by @rich, but the 404 Error: View not found [Name, Typ, Präfix]: user, html, site error is still remaining.
The only change I have made since quite a while is the activiation of the PhocaGallery User Control Panel, but nothing else.
Furthermore I have run under Kunena Tools the Diagnosis and have received only the following error: topicMissingMessages Test Failed 2 Problems Delete Broken Elements. Can that also play into the issue?