I started getting this error message yesterday while working in Sharepoint Designer. I would get the message whenever I would open a Sharepoint website. The solution to this problem is quite simple:

1) Open Internet Information Services (IIS) Manager.

2) Expand your computer list on the left then click on Application Pools.

3) Locate the application pool used by your Sharepoint website.

4) Right-click on the application pool and select Recycle.

You should now be able to connect to your Sharepoint website using Sharepoint Designer as usual.