... I DO NOT think it was a perfect kit...there maybe have been defects in the factory assembly of wings (maybe curing rates? maybe molds, I don't exactly know).

I toured the Aviastroitel factory in Moscow in 1999, when the AC-4 was being produced. All the structure was formed in a former Soviet aircraft factory outside Moscow, and then brought to Aviastroitel for finishing, completion and testing. All the structure was of good quality, but required a lot of finishing. So it could just be that a lot of finishing is required and those are the "defects" mentioned. After visiting Schleicher and Schempp-Hirth, the visit to Aviastroitel was fascinating. The Russians do an amazing job with very little.