"event.target.hostContainer is undefined" with signature fields

I got LC-Designer ES4 Vers. 11. In Acrobat XI (and possibly all other viewers) I have this problem:

Whenever I access a subform containing a signature field  (for example like this):

var oBlockSignatureEKB = xfa.resolveNode("Exportcontrol.blockWithSignatures.blockSignatureEKB");

console.println("setVisibilityOfSignatureEKB: [" + oBlockSignatureEKB.name + "]");

then I get in JavaScript-Debugger this error message

event.target.hostContainer is undefined

126:1

TypeError: event.target.hostContainer is undefined

126:1

Apparently the code quits working beyond this point.

Why is this?

How can I prevent it or get around this error?

Accessing subforms without a signature field I do not have this kind of problem at all.