Hi PowerUser,

I can confirm that .NET client 2.1.1 is the highest available for a 5.x server.
The new .NET PCL (Portable Class Library) 3.0.0 requires server 6.1.

That said, please let me stress that Ligtstreamer server 5.1.1 is a very old version and out of support since some time. The update to a newer version (6.1 or 7.0) is highly recommended to to take full advantage of all the bug fixes and enhancements introduced in these years of development.

Regards,
Giuseppe