I tried “SetProperty / SaveProperties”, but that does not work.
In order to access the properties in the background plugin I learned that I have to use “GetOptionsConfiguration”.
How can I save the new properties from backgroundplugin so that I can access the changed properties in the “Options Panel”?
The Property plugin sample shows how to use this, however it does only a read in the background (PropertyBackgroundPlugin, Init method)
https://doc.developer.milestonesys.com/html/index.html?base=samples/property.html&tree=tree_1.html
One thing to note is that a BackgroundPlugin per default does not run in the Smart Client environment, so check that it does. The sample use this code:
public override List<EnvironmentType> TargetEnvironments
{
get { return new List<EnvironmentType>() { EnvironmentType.Service, EnvironmentType.Administration, EnvironmentType.SmartClient, EnvironmentType.Standalone }; } // This plugin will run everywhere
}
Tip. See also - https://developer.milestonesys.com/s/article/debugging-techniques-for-Smart-Client-plugins
I tried it in the sample:
>> SetProperty (" MyNewBGProp ", Guid.NewGuid (). ToString ());
SaveProperties (false); <<
does not work from background plugin.
Is it even possible to write the properties from here?
You are right. I have now made the same observations. You will need to use the SaveOptionsConfiguration and GetOptionsConfiguration methods as a workaround.
I played around with it for a while. Although it is a little more effort in programming but it works. 
I feel bad because it did not work because I forgot one little thing..
LoadProperties(false);
_myProp = GetProperty("MyProp") ?? "null";
EnvironmentManager.Instance.Log(false, "Property read", "Value (global): " + _myProp, null);
I did not have the LoadProperties(false);
So simple to fix but I only saw it after consulting my colleagues..