I just stumbled upon a problem: in an application you can store rich content (i.e. MIME Richtext via the CKEditor) in an XPage. When I tried to upload an image via the CKEditor I got an exception page - saying nothing about the cause. A look in the error.log showed me the reason:
"[ ] Disk I/O is a restricted operation"
This happened also when trying to display content that contained an image (I formerly embedded the image via the Notes Client). The obvious behavior seems to be that the server executes a disk operation reading the stream for that embedded image - even in read mode. That was new to me.
So the signer of a database that does something like the described above must have "Sign/run restricted operations in LS/Java" set in the security settings of the server document (e.g. LocalDomainAdmins, LocalDomainServer).