Imported document nodes are not mapped to Organic Groups

I imported nodes with several custom fields & tags, using CSV import.
Al works fine apart from mapping the organic group node reference where the document node should be posted.
In my site, documents are are 'standard group post'.

I tried in my csv file the following data:
- the group node number, as a plain number
- the group node number between square brackets
- a url to the group

Nothing did the trick. I do not get any error messages.
This is an important feature for my (Drupal Commons based) website.

I saw a similar issue here, but it'd there since 2 years with no solution, so opened a new one.

Thanks upfront for suggestions and solutions