Hello @R.Bernhard ,
Please try the steps mentioned in below link:
404 FILE NOT FOUND SharePoint Error for Several Sites
We have 5 online SharePoint sites. I just set up a new SharePoint site two days ago and was working on permissions to restrict access to subfolders. Today, users are not able to see 3 different sites and I think it could be permission-related. I did make sure all the sites are listed under 'Active teams and groups' in the Admin Center. However, when I go to the site there is a 404 error.
For few of the sites, I am able to see the site if I add '/Shared%20Documents/Forms/AllItems.aspx' at the end of the address. However, I do not see the documents in the Document Library.
I am not sure what to do next. Your help is appreciated.