What is this 10 connection limit? Why am I seeing exceptions about this and how do I fix it?

The 10-connection limit is built into the WebSync Community edition server binaries (FM.WebSync.Server.dll). If you run your own WebSync server and you are seeing this exception then you are running the WebSync Community edition and you'll need to upgrade to the WebSync Enterprise edition. Download the WebSync Enterprise SDK and copy those binaries into your project, over-writing the Community edition binaries, and the restriction will go away.

If you do not run your own WebSync server, but instead use our WebSync Cloud service, and you are seeing a similar error please see this answer regarding Domain Keypair concurrency.