Croatian Airspace is divided over 6 Regional ATC centres, part of Croatia Control. I have had some success in getting QSLs from the Regional Centres. Mr. Gadzic from the Osijek Regional centre was so kind to return a QSL for NDB CE-372 (Osijek/Cepin) and NDB OSJ-422 (Osijek Airport).

QSL for NDB CE-372 and OSJ-422 at Osijek Airport

For who is interested, attached find some details about the Navaids at Osijek Airport.