I picked up a copy of the files and a backup of the database and put it out on a different server, but one which is also Windows Server 2008 R2 64 bit.
Here the portal works as expected - I can log on and edit content.
Some differences between the servers:
Server 1, where the login to DNN fails, has SharePoint Services websites installed.
Server 2 does not have SharePoint Services installed.
Other differences in Roles, Role Services and Features follow. Would any of these possibly interfere with ability to log on to DNN?
Fails on 1, succeeds on 2
Roles
Application Server (both)
System Services
Net.Msmq Listener Adapter on 1, not 2
Role Services
Message Queuing Activation is installed on 1, not 2
File Services (1, not 2)
System Services
LanmanServer Running Auto on 1
Role Service
File Server Installed
Web Server (IIS) (both)
System Services
Web Management Service WMSvc Running, Auto on 1 … Stopped, Manual on 2
Role Services (43 installed 1, 45 installed 2)
CGI 1 yes, 2 no
Custom Logging 1 no, 2 yes
ODBC Logging 1 no, 2 yes
FTP Extensibility 1 no, 2 yes
DNS Server 1 no, 2 yes (not relevant)
Features
5 of 41 installed 1, 4 of 42 installed 2
Windows PowerShell Integrated Scripting 1 no, 2 yes
Message Queuing, Message Queuing Services, Message Queuing Server 1 yes, 2 no
SNMP Services, SNMP Service, SNMP WMI Provicer 1 yes, 2 no