Export of JIRA issues in XML format fails in case when project does not have a Priority field available. It returns a HTTP 400 error code:

HTTP Status 400 - Not able to sort using field 'priority'.

XML export fails for both JQL and old-style queries.

What's interesting, it works for HTML view.

Steps to reproduce:
1) Create a project with a Priority field not associated to any issue type
2) Create an issue in the project
3) From Issue Navigator select Filters>Most important (it works), next Views>XMLOpen (it fails)

Sample URL:
/sr/jira.issueviews:searchrequest-xml/temp/SearchRequest.xml?jqlQuery=project+%3D+ABC+AND+resolution+%3D+Unresolved+ORDER+BY+priority+DESC&tempMax=1000

Workaround:
Associate Priority field with issue type.