Had a problem at work today where one of the staff could not open a site to edit it using SP Designer. One of the other people seemed to have more success (and they were running SP Designer with SP1) but they were still having problems.
The site was a restored site collection from a client’s environment (and the other instance of it we restored worked fine with SPD)… tried running an upgrade to SP1 for SPD on the erroring computer, but no dice… we even looked at the web sites to see if one of the app pools was off or the site had lost its application directories, IISReset (so we could check the IIS Control Panel Applet – problem and fix described here) but nothing unusual there…
We were at a loss, until one of the guys said he’d switched on Anonymous Authentication. Aha – I know that page – that’s where I stop people using SPD on a site by switching off Client Integration… and sure enough, it must have been switched off by accident when the Anon Auth was activated.