I think there's a reason why it occurs: The minimum dependency on .NET Framework for Npgsql is 4.6.1. The designer however uses 4.5.2.
~~If you have 4.6.2 installed, and you change in the llblgenpro.exe.config file the following line:
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2" />
to
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.2" />
does it then load the 4.1.1 npgsql driver?~~ (doesn't work)
In the next version of llblgen pro (5.7) we'll move to 4.6.2 or higher, to overcome this problem, as I was reminded this morning that google spanner makes a move to 4.6.2 soon, and now reading this I realize this might be the problem with 5.6.x.
I'll try to reproduce it here too.
(edit) hmm, setting that version to a higher version doesn't work. It's then indeed something related to npgsql.