Install issue

[rbnpi]

Trying out ImpressPages on a SynologyNAS. Install seems to go OK, database setup OK but at the end get this error, and First Page of install wizard (details of website name, timezone etc) is displayed again.

Unexpected error.Undefined index: websiteId in /volume1/web/mw/Ip/Internal/Install/Helper.php:374{"result":{"redirect":"index.php?step=2"},"id":null}

Install is in the folder mw which is the root directory for the site being used.
Not sure where to go from here.
(site is on a virtual apache server, and the url is in a local dns server but not yet published to internet)