SharePoint search no longer available for nearly all users

Hello Community,

we currently have a problem with a customers SharePoint site collection.

From all users, only one user account can search for documents on the site collection. No other user can search for documents anymore.

All the users who are searching for documents have the required permissions on both the library and document, when the users navigate manually to the document, they can open it - so there should be no permission problem here.

we also checked if the libraries and lists are searchable (in the library settings), which is positive.

We also triggered a reindex multiple times over the last few days, but that didn't really help.

I don't know where the problem is, since one user seems to be able to search for documents but nobody else can.

Can anybody give us some advice on what to check next or what this problem is?