It doesn't sound like it's because Anonymous doesn't have acess to create documents.
There is a new setting in the server document for 8.5 onwards, on the Security tab, right-hand side - "Sign agents or XPages to run on behalf of the invoker:"
Check the settings. All XPages and custom controls need to be signed by an ID that is authorised in that field.
If you can, it's worth installing 8.5.1 instead of 8.5. There were a lot of improvements/fixes for XPages in 8.5.1, as well as performance enhancements and XPages in Notes client.
Feedback response number WEBB7ZVCEZ created by Paul S. Withers on 01/20/2010