Success! Looks like we've fixed this one. According to our records the fix was applied for
a bug in our system
a recent build.
After project upgrade, border layout don't let you specify 'region' on linked classes
I have some components (one is an extension of a Container and one an extension of a Panel) that I had linked into a viewport that was using a border layout. After the project was upgraded, it no longer let's me specify a 'region' property for the linked classes (using an alias as the link). Then the border layout complains that there is no center region. I added a custom property called 'region' to the link and it seems to have fixed it, but I think you should not have to create a custom property for that.
Upgrade to 311. This was a bug in 309 and has since been resolved.