Connection problem on CtAPI.dll with a VB.net application on Citect 2018 R2

Pascal,
ensure all the dlls from CtApi.Win32.Redist.zip or CtApi.x64.Redist.zip as appropriate are in the same directory as your application.
You can find it in your installer forlder.

My application is running on a Citect client, it seems to me that these files are not mandatory, at least it was not mandatory with previous Citect versions. Is this the case with the 2018 R2 version?

Hi Pascal , these files have always been mandatory. Check in your VB code if it is referring to a folder location elsewhere on your machine. It is typical that customers just drop the ctapi.dll dependencies into the same folder where their client application is. It is now easier as we package exactly what you want into a zip and place it with the installation media.
There is a new mode for ctOpen to enable encrypted communications. See the online help for the topic "ctOpen".