Hi Vadim,

Indeed, I can confirm that .NET PCL Client 3.0.0 is incompatible specifically with the 6.0.x servers but should work fine with 5.x family servers.
This is because the new client version checker introduced with 6.0 that rejects the new PCL Client SDK.

So to recap, you can still use version PCL client 3.0.0 with your current server version, but you should be aware that you will have problems if the server is upgraded to version 6.0.
But considering that the 5.1.1 version is already very old I think it would make more sense to move directly to 7.0 (or at least 6.1).

Please let us know if you need any further clarifications.

Regards,
Giuseppe